A    TEXT-BOOK    OF 
QUANTITATIVE    CHEMICAL    ANALYSIS 


A    TEXT-BOOK 

OF 

Quantitative  Chemical 
|  Analysis 


BY 

ALEXANDER  CHARLES  GUMMING,  D.Sc. 

ii 

Lecturer  in  Chemistry r,  University  of  Edinburgh 
AND 

SYDNEY  ALEXANDER  KAY,  D.Sc. 

Assistant  in  Chemistry,  University  of  Edinburgh 


\    \  » V  *    V   < 

NEW   YORK 

JOHN    WILEY    &    SONS,    INC., 
432    FOURTH    AVENUE 

1913 


\  V  \  /V 


i... .-...-. 


PREFACE 

/ 

THIS  book  is  intended  primarily  for  University  and  College 
students,  and  in  planning  it  we  have  not  overlooked  the 
fact  that  those  who  study  Chemistry  as  a  subsidiary  subject 
usually  devote  so  short  a  time  to  it  that  it  is  impossible  for 
them  to  cover  any  comprehensive  course,  and  that,  even 
when  Chemistry  is  one  of  the  main  subjects  of  study,  the 
student,  as  a  rule,  has  a  strictly  limited  time  for  laboratory 
work. 

We  have  endeavoured,  therefore,  to  arrange  the  book  in 
such  a  manner  that  some  knowledge  of  the  principles  of 
Quantitative  Analysis  may  be  acquired  by  a  practical  study 
of  the  subjects  included  in  Parts  I.,  II.,  and  III.,  and  that  the 
further  requirements  of  those  who  are  making  a  special  study 
of  Chemistry  should  be  met  by  the  later  portions  of  the  book. 

Volumetric  Analysis  is  dealt  with  in  Part  II.  before  Gravi- 
metric Analysis,  partly  because  the  manipulation  is  easier, 
and  partly  because  the  exercises  in  Volumetric  Analysis 
present  a  greater  variety  than  those  in  simple  Gravimetric 
Analysis.  The  educative  value  of  volumetric  methods  is 
probably  greater  than  that  of  any  other  branch  of  analysis, 
and  we  are  of  opinion  that  a  student  should  receive  a 
thorough  training  in  Volumetric  Analysis^  even  if  the  time 
remaining  at  his  disposal  permits  of  little  or  no  gravimetric 
work. 

Most  of  the  typical  exercises  in  Parts  II.  and  III.  may 
be  performed  with  pure  substances,  but  it  is  desirable  that 
the  student  should  be  accustomed  from  the  commencement 
of  his  course  to  the  analysis  of  substances  of  "  unknown " 
composition.  The  serious  student  finds  that  this  enhances 


vi  PREFACE 

the  value  of  the  exercise,  whilst  the  occasional  student  who 
"  only  wants  to  know  the  method  "  has  his  attention  directed 
to  the  real  purpose  of  Quantitative  Analysis.  A  list  of 
solutions  suitable  for  analysis  is  given  in  the  Appendix.  In 
describing  typical  exercises,  care  has  been  taken  to  give  the 
practical  details  of  manipulation  as  fully  as  possible,  and 
where  full  details  are  not  given,  reference  is  invariably  made 
to  the  pages  where  they  may  be  found. 

In  Part  V.,  all  the  common  elements  and  radicals  are 
considered,  together  with  the  methods  for  their  separation 
and  determination.  As  the  arrangement  is  alphabetical 
and  copious  references  to  other  parts  of  the  book  are  given, 
it  is  hoped  that  this  section  will  prove  a  useful  index  to 
quantitative  methods  in  general. 

Water  analysis  is  included  because  it  always  appears  to 
interest  students,  and  because  it  affords  useful  exercises  in 
the  determination  of  substances  present  only  in  traces. 

In  order  to  avoid  constant  repetition  of  particulars  in 
regard  to  the  concentration  of  reagents,  it  has  been  assumed 
throughout  the  book  that,  unless  the  contrary  is  stated,  the 
concentration  of  a  reagent  is  that  specified  in  the  Appendix. 
The  concentrations  usually  recommended  for  indicator  solu- 
tions are  such  that  even  "  a  few  drops  "  is  often  more  than 
ought  to  be  used.  The  concentrations  recommended  in  the 
Appendix  are  so  chosen  that  I  c.c.  of  the  indicator  is  the 
normal  amount  required,  and  throughout  the  book  it  is 
assumed  that  these  dilute  indicator  solutions  are  used. 

All  the  diagrams  have  been  specially  drawn  for  the  book 
— in  a  large  number  of  cases  from  original  photographs  of 
the  apparatus. 

We  desire  heartily  to  acknowledge  our  indebtedness  to 
Dr  Leonard  Dobbin,  whose  helpful  counsel  has  been  at  our 
disposal  during  the  preparation  of  the  manuscript. 

CHEMISTRY  DEPARTMENT, 

UNIVERSITY  OF  EDINBURGH, 
October,  1913. 


CONTENTS 


PART   L— GENERAL   PRINCIPLES. 


Introductory 

Volumetric        and        Gravimetric 

Methods         . 

The  Balance  and  Weighing  . 
Calibration  of  Weights  . 
Notes  on  General  Apparatus          . 


PAGE 

I 

2 

4 

10 

14 


Preparation  of  the  Substance  for 

Analysis          .         .         .         .16 
Solution  of  the  Substance       .         .     20 

Evaporation 21 

Precipitation .....     22 
Filtration 23 


PART   II.— VOLUMETRIC   ANALYSIS. 


28 


Introductory  ..... 
The  Measurement  of  Volumes  of 

Liquids 30 

Standardisation  of  Instruments      .     32 
General  Notes  on  the  Preparation 

of  Standard  Solutions     .         .     41 


ACIDIMETRY  AND  ALKALIMETRY. 

Introductory 44 

The  Use  of  Indicators   .         .         .44 
Standard  Hydrochloric  Acid.         .     47 
Standard  Sulphuric  Acid        .         .     52 
Standard  Sodium  Hydroxide          .     52 
Analyses    involving    the    Use    of 
Standard  Acid  and  Alkali — 
Acetic  Acid  in  Vinegar       .         .     55 

Borax 55 

Solubility  of  Lime  in  Water       .     56 

Mercury 56 

Oxide  and  Carbonate  in  Quick- 
lime     57 

Acidic     Radical     in     Salts     of 

Heavy  Metals     .  .58 

Ammonia  (Indirect  Method)      .     58 
Ammonia  (Direct  Method)         .     59 

Nitrate 60 

Persulphate        .        .         .         .61 
Standard  Baryta  Solution      .         .     61 
Standard  Lime-Water    .        .         .63 
vii 


67 
67 
68 


STANDARD  POTASSIUM  PERMAN- 
GANATE AND  DICHROMATE. 
Decinormal     Potassium    Perman- 
ganate   .  .         .         .     ( 
Analyses    involving    the    Use    of 

Standard  Permanganate — 
Oxalic  Acid  and  Oxalates  . 
Peroxides  .... 
Nitrite        .... 
Calcium     .         .         .         .         .69 

Nitrate 70 

Decinormal  Potassium  Dichromate     71 
Analyses     involving    the    Use   of 
Standard     Permanganate     or 
Dichromate  Solutions — 
Iron  in  Iron  Wire      .         .         -74 
Iron     in     Ferrous    and    Ferric 

Compounds         .         .         .76 
Total  Iron  in  a  Mineral      .         .     80 
Separate  Determination  of  Fer- 
rous and  Ferric  Iron  in  a 
Mineral       .... 
Iron  in  Black  Ink 
Iron  and  Chromium  in  Chrome 
Iron  Ore     , 


82 
83 


STANDARD  IODINE  AND  STANDARD 
SODIUM  THIOSULPHATE. 

Decinormal  Sodium  Thiosulphate      85 
Decinormal  Iodine         .         .        ,88 


viii 


CONTENTS 


PART  II.— VOLUMETRIC  ANALYSIS— continued. 


Analyses  involving  the  Use  of 
Standard  Iodine  and  Standard 
Sodium  Thiosulphate — 

Copper 89 

Sulphurous  Acid  and  Sulphites  .     91 
Hydrogen  Sulphide  .  .91 

Peroxides,  Chromates,  Chlorates    92 
Available  Chlorine  in  Bleaching 

Powder  .  .  .  -94 
Tin  in  an  Alloy  .  .  .95 
Tin  in  an  Ore  .  .  .  .  96 

STANDARD  SILVER  NITRATE  AND 
POTASSIUM  THIOCYANATE. 

Decinormal  Silver  Nitrate      .         .  98 
Analyses    involving    the    Use    of 

Standard  Silver  Nitrate — 

Chloride  and  Bromide        .         .  99 

Chloride  in  Barium  Chloride      .  99 

Cyanide 99 


PAGE 
100 


Hydrocyanic  Acid 

Decinormal     Silver    Nitrate    and 

Decinormal   Potassium  Thio- 

cyanate 101 

Analyses    involving    the    Use    of 
Standard   Silver   Nitrate  and 
Standard  Thiocyanate — 
Chloride,  Bromide,  and  Iodide  .   103 

Chlorate 104 

Silver 104 

Mercury 104 

Total     Chlorine    in    Bleaching 

Powder       ....  105 

VARIOUS  VOLUMETRIC  PROCESSES. 

Available  Chlorine  in  Bleaching 
Powder  by  means  of  Standard 
Sodium  Arsenite  Solution  .  107 

Zinc  by  means  of  Standard  Sodium 

Sulphide  Solution  .  .  .  108 


PART  III.— GRAVIMETRIC  ANALYSIS. 


Introductory  ....  109 

Notes  on  Apparatus      .         .         .  no 

The  Gooch  Crucible  .        .        .112 

The  Rose  Crucible    .        .        .115 

The    Ignition    and  Weighing    of 

Precipitates    .         .         .         .116 

TYPICAL  GRAVIMETRIC  EXERCISES. 

Water  in  Magnesium  Sulphate 

Heptahydrate  .  .  .123 

Water  in  Barium  Chloride  Crystals  124 

Anhydrous  Disodium  Hydrogen 
Phosphate  in  the  Crystalline 
Salt 124 

Iron  in  Iron  Ammonium  Alum  by 

Ignition  .  .  .  .125 

Other  Examples  of  Analysis  by 

Ignition  .  .  .  .126 


Iron  as  Ferric  Oxide     .         .  .127 

Aluminium  as  Oxide      .         .  .130 

Sulphate  as  Barium  Sulphate  .   131 

Chloride  as  Silver  Chloride  .  .133 

Magnesium  as  Pyrophosphate  .   135 

Zinc  as  Oxide        .         .         .  .137 

Copper  as  Cupric  Oxide         .  .  139 

Copper  as  Cuprous  Sulphide  .   141 

Calcium  as  Oxalate        .         .  .   143 

ELECTROLYTIC  METHODS. 

General 145 

Copper    (with     Stationary     Elec- 
trodes)     148 

Cadmium       .....  149 

Copper  (with  a  Rotating  Cathode)  150 

Nickel 153 

Lead  as  Dioxide     .         .        .         .153 


PART  IV.— COLORIMETRIC  METHODS. 


Introductory 
Iron 
Copper . 


155 
156 
158 


Ammonia      .        .         .         .        .159 

Lead     .         .         .        .        .         .161 

Manganese  .        .         t  i§2 


CONTENTS 


ix 


PART  V.— SYSTEMATIC  QUANTITATIVE  ANALYSIS. 


Aluminium 165 

Ammonium  .....  167 

Antimony 167 

Arsenic  .....  168 

Barium 169 

Bismuth 170 

Bromide 173 

Cadmium 174 

Calcium 175 

Carbonate 175 


Chlorate 

Chloride 

Chromium     . 

Chromate  and  Bichromate 

Copper  .... 

Iron   .... 


181 
181 
182 
182 
183 
185 


PAGE 

Lead     .         .         .        ...         .187 

Magnesium 188 

Manganese    .         .         .         .         .189 
Mercury         .....  192 

Nickel 195 

Phosphate 196 

Potassium  and  Sodium  .         .        .  199 
Silica  and  Silicates         .         .        .  206 

Silver 211 

Sodium          .         .         .         .         .211 
Sulphate        .         .         .        .         .211 

Sulphide 212 

Tin       ...        .        .        .  212 

Water 213 

Zinc 216 


PART  VI.— THE  ANALYSIS  OF  SIMPLE  ORES  AND 
ALLOYS. 


Silver  Coin    . 
German  Nickel  Coin 
Solder   . 

Bronze  .         .        , 
Fusible  Alloy 
Limestone  or  Dolomite 
Insoluble  Silicate  . 


220 
221 
223 
224 
226 
228 
232 


Glass 236 

Iron  Pyrites 239 

Copper  Pyrites       .         .         .         .241 

Galena 244 

Zinc  Blende 245 

Pyrolusite  or  Manganite        .         .  248 
Superphosphate  Manure        .         .  249 


PART  VII.— GA^  ANALYSIS. 


Introductory 253 

Collection  of  a  Sample  of  Gas  for 

Analysis 254 

GAS  ANALYSIS  WITH  THE  HEMPEL 
APPARATUS. 

The  Gas-Burette  .  .  .  .256 
Absorption  Pipettes  .  .  .  259 
Reagents  used  in  Absorption 

Pipettes  »  .  .  .  .  261 
Manipulation  of  Apparatus  .  .  264 
Analysis  of  a  Gaseous  Mixture  .  266 


GAS  ANALYSIS  WITH  THE  ORSAT 
APPARATUS. 

The  Orsat  Apparatus     .         .         .  269 
Collection  of  the  Sample       .         .271 
Analysis  of  the  Gas        .         .         .271 
Determination    of    Hydiogen     l.y 
Combustion   in  Contact  with 
Palladium       ....  272 

ANALYSES  INVOLVING  THE  USE  OF  A 
LUNGE  NITROMETER. 

The  Lunge  Nitrometer          .         -275 
Nitrogen  in  a  Nitrate  or  Nitrite    .  275 


CONTENTS 


PART  VII.— GAS  ANALYSIS— continued. 


Hydrogen  Peroxide 
Zinc  Dust 


PAGE 
,  277 
,  278 


DETERMINATION  OF  GASES  PRESENT 

ONLY  IN  TRACES. 
General         .        ,        .        .        .  279 


Sulphur  in  Coal  Gas      ..       . 
Atmospheric  Carbon  Dioxide 
Hydrogen  Sulphide  in  Coal  Gas 
Hydrocyanic  Acid  in  Coal  Gas 
Sulphur  Dioxide  in  Flue  Gases 


PAOK 

.  280 
,  282 

,285 
,  285 
,  285 


PART  VIII.— WATER  ANALYSIS. 


Introductory 


.  286 


PHYSICAL  AND  CHEMICAL  METHODS 
OF  EXAMINATION  AND  ANALYSIS. 

Collection  of  Samples  of  Water  .  289 
Physical  Examination  .  .  .  290 
Chemical  Examination — 

Total  Solids  .  .  .  .292 
Free  and  Albumenoid  Ammonia  293 
Reducing  Power  .  .  .  296 
Chloride 298 


Nitrite  .  .  .  .  .  299 
Nitrate  .  .  .  .  .  300 
Phosphate  .  .  .  .  .  302 

Hardness 302 

Relative  Acidity  and  Alkalinity    307 

Lead 311 

Action  of  Water  on  Lead  .  .312 
Iron  .  .  .  .  .  313 

Zinc  and  Copper        .         .         .313 
Saline  Constituents    .        .        .313 
Significance    of    the     Results    of 

Analysis  of  a  Potable  Water  315 


PART   IX.- QUANTITATIVE   ANALYSIS   OF   ORGANIC 
SUBSTANCES. 


Combustion  Apparatus .         .         .318 

Preparation  of  the  Combustion 

Tube 322 

Combustion  of  a  Solid  Substance 
containing  Carbon  and 
Hydrogen  .  .  •*  .  324 

Combustion  of  a  Liquid         .         .  327 


Modification  if  Nitrogen  is  Present  328 
Modification     if    Sulphur     or     a 

Halogen  is  Present  .  .  329 
Nitrogen  by  Dumas'  Method  .  329 
Nitrogen  by  Kjeldahl's  Method  .  334 
Chlorine,  Bromine,  and  Iodine  .  335 
Sulphur 336 


PART   X.— THE   DETERMINATION   OF   MOLECULAR 

WEIGHTS. 


Victor  Meyer's  (Constant  Pres- 
sure) Method  .  .  -339 

Lumsden's  (Constant  Volume) 

Method 342 

The  Freezing-Point  Method .         .  345 


Beckmann's  Boiling-Point  Method  353 
Modification     with      Electrical 

Heating      .         ^        ,         .  355 
Landsberger's         Boiling  -  Point 

Method.   .     .  356 


CONTENTS 


APPENDIX. 


List  of  Common  Reagents     . 

Special  Reagents  . 

Indicator  Solutions 

Standard  Solutions  for  Analysis 

Typical  Analyses  . 

Density    and     Concentration 

Various  Acids 
Density    and     Concentration 

Various  Alkalis 


PAGE  PAGE 

,  361  Density     and      Concentration      of 
,  363  Aqueous  Alcohol    .         .         .  370 

,  364  Weight  of  i  litre  of  Various  Dry 
.  364  Gases     .... 

.  366  Vapour  Pressure  of  Water     . 

of  Vapour     Pressure    of    Potassium 
.  368  Hydroxide  Solutions      .         .371 

of  Table  of  Logarithms     .         .         -372 

.  370  Atomic  Weights    ....  374 


371 

371 


INDEX  OF  SEPARATIONS 


37S 


INDEX 


377 


QUANTITATIVE  CHEMICAL 
y    ANALYSIS 

/ 

PART     I 

GENERAL  PRINCIPLES 

WHEN  the  examination  of  any  substance  is  undertaken 
for  the  purpose  of  determining  the  respective  amounts  of  any 
of  its  constituents,  the  investigation  is  known  as  quantitative 
analysis.  The  problem  may  be  a  simple  or  a  complex  one, 
depending  on  the  nature  of  the  substance,  and  on  whether 
a  complete  or  only  a  partial  analysis  is  required.  For  many 
purposes,  it  is  not  necessary  to  ascertain  the  amounts  of  all 
the  constituents  of  a  substance ;  it  may  be  of  importance 
to  determine  the  amount  of  only  one  of  them.  It  is 
comparatively  simple  to  determine,  for  example,  the  amount 
of  iron  in  an  ore,  the  amount  of  carbon  dioxide  in  a  sample 
of  air,  or  the  amount  of  chloride  in  a  water  supply.  On  the 
other  hand,  it  may  be  necessary  to  make  a  complete  analysis 
of  a  complex  ore  or  rock,  containing  as  many  as  ten  or 
twenty  constituents,  or  to  carry  out  a  detailed  investigation 
of  a  sample  of  water.  The  complexity  of  an  analysis  depends,' 
however,  as  much  on  the  nature  of  the  constituents  as  on 
their  number,  and  the  determination  of  the  amount  of  even 
a  single  constituent  may  involve  a  lengthy  and  refined 
investigation,  demanding  the  highest  skill  on  the  part  of 
the  chemist. 

There  are  usually  several  distinct  methods  for  the 
determination  of  one  and  the  same  substance,  all  of  which 
may  not  be  applicable,  however,  to  the  particular  case.  The 

A 


$  :  tastfEfcAL  PRINCIPLES 

procedure  adopted  is  sometimes  a  matter  of  convenience, 
but  the  choice  of  the  best  method  more  often  requires 
careful  consideration. 

The  gravimetric  method  of  analysis  in  most  cases  involves 

(1)  the  separation  of  the   constituents  of  the   substance  in 
the   form  of  insoluble    compounds  of   known  composition ; 

(2)  the   determination  of  the  weight  of  the   compounds  so 
obtained. 

The  volumetric  method  of  analysis,  on  the  other  hand,  is 
based  on  the  use  of  a  reagent  of  known  concentration  and  on 
the  measurement  of  the  volume  of  this  reagent  required  to 
complete  the  chemical  change  involved. 

A  fundamental  distinction  between  the  two  methods  is, 
that  in  gravimetric  analysis  the  constituent  which  is  to  be 
determined  must  first  be  separated  from  all  the  other  con- 
stituents of  the  substance ;  whereas,  in  volumetric  analysis^ 
the  complete  isolation  of  the  constituent  is  very  frequently 
unnecessary,  and  one  of  the  constituents  of  a  substance  can 
often  be  rapidly  and  accurately  determined  in  presence 
of  all  the  others,  thus  enormously  simplifying  the  analytical 
process. 

Most  substances  can  be  determined  either  gravimetrically 
or  volumetrically.  In  the  systematic  treatment  of  the 
subject,  it  is  convenient  to  consider  gravimetric  and  volu- 
metric methods  separately  ;  but  in  practice  the  two  methods 
of  procedure  are  frequently  combined,  in  order  that  the 
analysis  may  be  completed  as  rapidly  and  as  accurately  as 
possible.  When  a  complete  analysis  of  a  complex  substance 
has  to  be  made,  the  constituents  must,  as  a  rule,  be  separated 
from  one  another  before  the  amount  of  each  can  be 
ascertained,  and  in  such  a  case  gravimetric  methods  are 
usually  employed ;  whereas,  if  only  a  partial  analysis  is 
required,  involving,  it  may  be,  only  one  of  the  constituents, 
volumetric  methods  are  often  applicable.  The  latter  are 
almost  invariably  more  expeditious  than  gravimetric  methods, 
and,  in  analysis  for  technical  purposes,  where  economy  of 
time  is  often  imperative,  volumetric  methods — not  necessarily 
less  accurate  than  gravimetric — are  used  as  far  as  possible. 

As  an  example  in  illustration  of  some  of  the  foregoing 
principles,  two  methods  of  determining  the  respective 


GENERAL  PRINCIPLES  3 

amounts   of  iron   and   aluminium  in   a   solution   containing 
ferric  and  aluminium  chlorides  may  be  briefly  outlined. 

(1)  In  order  to  accomplish  this  by  gravimetric  methods 
alone,  the  iron  and  aluminium  must  be  separated  by  adding 
an  excess  of  sodium  hydroxide  to  a  weighed  or  measured 
portion  of  the  solution.     The  precipitate,  which  consists  of 
ferric     hydroxide,    is    filtered;     the   filtrate     contains    the 
aluminium  as  sodium  aluminate. 

The  precipitate,  which  is  contaminated  with  alkali 
hydroxide,  is  dissolved  in  nitric  acid,  and  ammonia  is 
added  in  order  to  reprecipitate  the  ferric  hydroxide.  The 
latter,  after  filtration,  is  converted  into  ferric  oxide  which 
is  weighed. 

The  filtrate,  containing  the  sodium  aluminate,  is  acidified 
with  hydrochloric  acid,  and  the  aluminium  is  precipitated  as 
aluminium  hydroxide  by  adding  ammonia.  The  precipitate 
is  filtered  and,  by  heating  to  a  high  temperature,  is  converted 
into  alumina  which  is  weighed. 

From  the  weights  of  ferric  oxide  and  alumina,  the 
respective  amounts  of  iron  and  aluminium  in  the  solution 
can  then  be  calculated. 

(2)  By   a  combination   of    gravimetric    and    volumetric 
methods,   which   in  this   case  is   much  to  be  preferred,   no 
separation    of   the   iron   and   aluminium    is    necessary;    the 
procedure  is  accordingly  simpler  and  more  expeditious,  and 
accurate  results  are  more  readily  obtained. 

The  iron  and  aluminium  are  precipitated  together  as 
hydroxides  by  adding  ammonium  chloride  and  ammonia  to 
a  weighed  or  measured  portion  of  the  solution,  and  the 
precipitate,  by  heating  strongly,  is  converted  into  a  mixture 
of  ferric  oxide  and  alumina,  which  is  weighed,  The  mixture 
of  ferric  oxide  and  alumina  is  then  dissolved  (or  another 
measured  portion  of  the  original  solution  is  taken),  and  the 
iron  in  the  solution  is  determined  volumetrically.  The 
volumetric  process  consists,  briefly,  in  reducing  the  ferric 
salt  to  the  ferrous  state  by  means  of  hydrogen  sulphide  or 
other  suitable  reducing  agent,  and  in  then  determining  the 
amount  of  iron  present  by  means  of  a  solution  of  potassium 
permanganate  of  known  concentration.  The  aluminium  does 
not  interfere  with  the  volumetric  determination  of  the  iron. 


4  GENERAL  PRINCIPLES 

It  is  then  easy  to  calculate  how  much  ferric  oxide  is 
present  in  the  mixture  of  ferric  oxide  and  alumina,  and  the 
difference  between  the  total  weight  of  the  mixed  oxides 
(which  has  already  been  determined  gravimetrically)  and  the 
weight  of  the  ferric  oxide,  is  the  weight  of  the  alumina.  The 
respective  amounts  of  iron  and  aluminium  in  the  original 
solution  can  then  be  calculated. 

The  Balance. 

For  accurate  analytical  work  a  suitable  balance,  capable 
of  supporting  a  maximum  load  of  100  to  200  grams  in  each 
pan,  is  indispensable.  It  is  important  that  the  maximum 
load,  whatever  it  may  be,  should  not  be  exceeded.  With  a 
good  balance,  properly  adjusted  and  used,  very  accurate 
measurements  can  be  made.  For  example,  it  is  possible  to 
distinguish  between  two  masses  of  about  10  grams  each 
when  they  differ  in  weight  by  only  01  milligram,*'.*?,  by  i 
part  in  100,000.  A  balance  is,  therefore,  a  delicate  instru- 
ment of  precision,  and  the  greatest  possible  care  must  be 
taken  in  using  it.  The  rules  regarding  the  use  of  the  balance 
must  be  carefully  read,  and  thereafter  strictly  adhered  to. 

When  weighing  in  a  comparatively  rough  fashion,  it  is 
generally  assumed  that  equipoise  is  established  when  the 
excursions  of  the  pointer  towards  either  side  of  the  mid-point 
of  the  scale  are  of  equal  amplitude.  There  are  reasons, 
however,  why  this  method  is  not  adopted  in  accurate 
work. 

(1)  The    resting-point,   or    zero-point,   of    the    unloaded 

balance,  i.e.  the  position  which  the  pointer  would 
apparently  take  up  if  the  oscillating  beam  were 
allowed  to  come  to  rest,  seldom  coincides  exactly 
with  the  mid-point  of  the  scale. 

(2)  Since  the  oscillating  beam,  if  left  to  itself,  ultimately 

comes  to  rest,  the  amplitude  of  each  oscillation,  even 
when  equipoise  is  established,  is  less  than  that  of 
the  preceding  one.  It  follows  that,  if  an  excursion 
of  the  pointer  to  the  left  is  equal  to  the  preceding 
one  to  the  right,  the  weight  on  the  right  is  greater 
than  that  on  the  left  (assuming  the  zero-point  to 
coincide  with  the  mid-point  of  the  scale). 


USE  OF  THE  BALANCE  5 

Routine  Method  of  Weighing. 

In  making  a  weighing,  accurate  to  o-i  milligram  (o-oooi 
gram),  the  following  method  should  be  used  :  — 

(i)  Find  the  zero-point  of  the  unloaded  balance. 

Release  the  beam  gently,  and  if  necessary  set  it  oscillat- 
ing (by  wafting  air  down  upon  one  of  the  pans)  so  that  the 
pointer  moves  through  about  five  scale  divisions  on  either 
side  of  the  middle  point.  Close  the  balance-case,  and, 
neglecting  the  first  complete  oscillation  (two  excursions  of 
the  pointer),  carefully  observe  and  note  down  the  next  three 
extreme  positions  of  the  pointer,  two  observations  being 
made  on  one  side  and  one  on  the  other  side  of  the  mid-point 
of  the  scale.  Assume  the  scale  to  be  numbered  from  the 
extreme  left  towards  the  right,  i.e.  from  o  to  20,  the  mid- 
point being  10,  and  estimate  tenths  of  the  scale  divisions. 

If,  for  example,  the  observations  were 

Left.  Right. 

(i)  5-0 

(2)    15-8 

(3)  5-4 

the  turning-point  on  the  left,  corresponding  with  the  point 
15-8  on  the  right,  is  the  mean  of  5-0  and  5-4,  i.e.  5-2,  and  the 
resting-point  is  therefore 


Repeat  the  observations  several  times.  The  results  should 
not  differ  by  more  than  one  or  two  tenths  of  a  scale  division, 
and  the  mean  is  taken  as  the  zero-point  of  the  balance.  As 
the  zero-point  is  frequently  subject  to  slight  fluctuations,  it 
should  be  determined  before  each  set  of  weighings  is 
commenced. 

(2)  Place  the  vessel  to  be  weighed  on  the  left  pan  of  the 
balance  and  proceed  to  counterpoise  it.  It  is  best  to  begin 
with  a  weight  that  will  probably  prove  too  heavy,  as  this 
may  save  time  in  the  end.  For  example,  if  the  weight  of  the 


6  GENERAL  PRINCIPLES 

vessel  is  thought  to  lie  between  15  and  20  grams,  the  latter 
weight  is  placed  on  the  right  scale-pan.  If,  on  releasing  the 
beam,  the  2o-gram  weight  is  seen  to  be  too  much,  it  is 
replaced  by  a  lo-gram  weight,  and  the  necessary  smaller 
weights  are  added  in  regular  succession  until  finally  it  is 
found  that,  for  example,  16-46  grams  is  too  little,  whilst  16-47 
grams  is  too  much. 

In  place  of  the  inconveniently  small  milligram  weights,  it 
is  preferable,  at  this  stage,  to  use  a  rider,  which  weighs  o-oi 
gram,  whilst  its  effective  weight  depends  on  its  position  on 
the  divided  beam.  After  some  experience,  it  will  be  found 
possible  approximately  to  estimate,  by  observing  the  extent 
and  rapidity  of  the  oscillation,  what  additional  weight  is 
required  to  establish  equilibrium.  If,  for  instance,  it  is  found 
that  with  16-46  grams  on  the  pan  the  pointer  is  deflected 
slowly  to  the  right  whilst  with  16-47  grams  it  is  deflected 
much  more  rapidly  and  to  a  greater  extent  to  the  left,  the 
weight  of  the  vessel  is  nearer  16-46  than  16-47  grams. 

Place  the  rider,  then,  on  the  beam,  in  such  a  position  that 
'equipoise  is  nearly  established — for  example,  at  division  3. 
Close  the  balance-case  and  determine  the  resting-point. 
Suppose  it  is  found  to  be  9-6. 

(3)  Find  the  "  sensitiveness "  of  the  balance,  i.e.  the  dis- 
placement of  the  resting-point  produced  by  an  alteration  of 
I  milligram  : — Alter  the  position  of  the  rider  by  an  amount 
corresponding  to  i   milligram — in  such  a  direction  that  the 
resting-point  is  shifted  to  the  other  side  of  the  zero-point — 
and  again  determine  the   resting-point.     Suppose  it   to   be 
1 1- 1  when  the  rider   is   at  division  2.     The  sensitiveness   is 
then  equal  to 

11-1-9-6=1-5  scale  divisions. 

(4)  Now  calculate,  as  follows,  the  alteration  of  the  weight 
necessary  to  counterpoise  the  vessel  exactly : — 

The  zero-point — using  the  figures  assumed  in  the  fore- 
going— is  10-5,  and  the  resting-point  with  a  load  of  16-462 
grams  is  u-i.  The  vessel  weighs,  therefore,  more  than 
16-462  grams,  the  additional  amount  being  equal  to  that 
necessary  to  displace  the  resting-point  from  n-i  to  10-5,  or 
0-6  of  a  scale  division.  Since,  however,  1-5  scale  divisions  cor- 


USE  OF  THE  BALANCE  7 

responds  with  I  milligram,  0-6  scale  division  is  equivalent  to 

06 

—  =  0-4  milligram. 

The  weight  of  the  vessel  is  therefore  16-4624  grams. 

The  complete  weighing  thus  involves  the  determination 
of  three  resting-points — the  first,  that  observed  with  the 
empty  balance;  the  second,  after  approximately  counterpois- 
ing; the  third,  after  making  an  alteration  of  ±  I  milligram. 
All  the  observations  made  in  the  above  example  are  shown 
below : — 

Resting-points. 


Mean 
Resting-points 


(1) 

Unloaded 
balance. 

(2) 

With  load  of 
16-463  grams. 

(3) 
With  load  of 
16-462  grams. 

5-0 

4-4 

6-7 

15-8 
5'4 

14-7 

4-6 

15-3 
7-0 

6-9    15-3 

IM 

5-2    15-8 
10-5 

4-5  147 
9-6 

iveness 
ion 

=     IM—    9-6 
=     IM-IO-5 

=    1-5  scale  divisions 

=  °'6    „            „ 

Additional  weight  =  =  0-4  milligram 

Weight  of  vessel     =    16-462 +  0-4  milligram 
=    16-4624. 


Determination  of  the  Sensitiveness  of  the   Balance  -with 
different  Loads. 

It  is  evident  from  the  foregoing  that  the  process  of 
weighing  may  be  considerably  shortened  if  the  sensitiveness 
of  the  balance  is  already  known.  The  sensitiveness  varies, 
however,  with  the  load  on  the  balance.  According  to  theory, 
it  increases  with  the  load,  since — provided  the  three  knife- 
edges  are  in  the  same  plane — the  centre  of  gravity  of  the 
balance  is  raised  by  increasing  the  load,  and  is  brought 
nearer  the  point  of  support.  In  practice,  however,  it  is  found 
that  the  sensitiveness  sometimes  increases  and  sometimes 
decreases  slightly  as  the  load  is  increased.  This  is  the 
result  of  slight  flexure  of  the  beam,  increased  friction  at  the 
knife-edges,  etc. 


8  GENERAL  PRINCIPLES 

Determine,  then,  the  sensitiveness  with  5,  10,  20,  and  50 
grams  in  each  pan.  There  is  some  advantage  is  placing  an 
excess  of  5  mg.  on  the  left  scale-pan  in  order  that  the 
counterpoise  may  be  obtained  with  the  rider  near  the  middle 
of  the  right  arm  of  the  balance.  The  resting-point  is  then 
determined  with  the  rider  in  two  positions,  differing  by  I 
or  by  2  milligrams,  and  so  chosen  that  the  resting-points  are 
found  on  opposite  sides  of  the  zero-point.  From  the  observa- 
tions calculate  the  sensitiveness  with  each  load,  that  is,  the  dis- 
placement of  the  resting-point  produced  by  an  alteration  of 
±  i  milligram,  and  keep  a  record  of  the  results  in  a  note-book. 

Having  determined  the  sensitiveness  in  this  way,  once  for 
all,  the  process  of  weighing  resolves  itself  into  the  following 
two  operations: — 

(1)  Find  the  zero-point  of  the  balance. 

(2)  Counterpoise  the  object  to  the  nearest  milligram  and 

find  the  resting-point. 

The  fraction  of  a  milligram  that  must  be  added  (or  sub- 
tracted) to  complete  the  counterpoise  is  then  calculated.  If, 
for  example,  the  zero-point  is  10-2,  the  resting-point  9-7  with 
a  weight  of  5-826,  and  the  sensitiveness  1-7,  then  the  correct 

weight  is  -     2~       =  —  =  -3  milligram  less  than  the  weight 
on  the  balance-pan;  that  is,  5-8257  grams. 


Rules  to  be  Observed  in  Using  the  Balance. 

It  should    be  remembered   that  one  incorrect  weighing 
spoils  the  whole  analysis. 

1.  The  object  to  be  weighed  must  be  at  room  temperature. 

If  it  has  been  heated,  sufficient  time  must  be  allowed 
for  cooling.  The  time  required  to  attain  the  room 
temperature  varies  with  the  size,  etc.,  of  the  object, 
but  as  a  rule  twenty  minutes  is  sufficient. 

2.  Glass  vessels,  after  handling,  should  be  wiped  with  a 

soft,  dry  cloth,  and  then  left  in  the  balance-room  for 
at  least  twenty  minutes  before  weighing.  This  is 
necessary  more  especially  in  the  case  of  large  vessels, 


RULES  FOR  USE  OF  BALANCE  9 

such  as  flasks,  U-tubes,  etc.,  the  weight  of  which  may 
vary  by  several  milligrams  according  to  the  conditions 
under  which  they  are  weighed. 

3.  Liquids  and  volatile  solids  must  be  weighed  in  a  closed 

vessel,  such  as  a  stoppered  bottle.  If  the  vapour  is 
corrosive,  the  vessel  should  not  be  opened  in  the 
balance-room. 

4.  Never  place  the  substance  to  be  weighed  directly  on 

the  balance-pan,  but  place  it  on  a  watch-glass  or 
scoop,  or  in  a  weighing-bottle.  When  it  is  necessary 
to  add  more  of  a  solid  or  liquid,  the  operation  must 
be  performed  outside  the  balance-case. 

5.  Sit  opposite  the  middle  point  of  the  balance. 

6.  Release    the    beam    (and    arrest    it)    gently,  and,   if 

necessary,  set  the  balance  swinging  by  wafting  air 
downwards  on  one  of  the  pans.  The  pans  must  not 
swing  from  side  to  side. 

7.  Find    the   resting-point   of  the   balance   before   each 

weighing. 

8.  The  pointer  should  move  through  four  or  five  divisions 

beyond  the  mid-point  of  the  scale.  The  pans  must 
on  no  account  be  touched  while  the  beam  is  swinging. 

9.  Lift  the  weights  only  with  the  forceps. 

10.  Before    placing    additional   weights    on   the    pan,   or 

removing  any  therefrom,  the  balance  must  be 
arrested. 

11.  Always   close  the   balance-case  when  starting  to  use 

the  rider. 

12.  Note  down  the  weight  in  a  note-book  (not  on  a  loose 

slip  of  paper,  which  is  apt  to  be  lost)  before  removing 
the  weights  from  the  pan  by  noting  the  empty  places 
in  the  box,  and  check  the  weights  as  they  are  being 
removed  from  the  pan. 

13    Never  leave  anything  on  the  pans  when  weighing  is 
finished. 

14.  Close  the  balance-case  when  finished. 


10  GENERAL  PRINCIPLES 

Exercises  in  Weighing. 

Read  the  description  given  of  the  routine  method  of 
weighing  (pages  5  to  9)  and,  after  careful  study  of  the  rules 
to  be  observed  in  using  the  balance,  proceed  to  practise  the 
following  exercises,  in  order  to  become  familiar  with  the 
balance,  and  the  method  of  using  it.  Make  notes  of  your 
observations  and  submit  them  for  inspection. 

(1)  Find    the    zero-point   of  the    balance.      Repeat   the 

operation  four  times  (at  least),  arresting  the  balance 
after  each.  Keep  a  record  of  your  observations,  as 
shown  on  page  7. 

(2)  Find  the  sensitiveness  of  the  balance  with   loads  of 

(a)  5  grams,  (b)  10  grams,  (c)  20  grams,  (d)  50  grams, 
in  each  pan. 

Tabulate  the  results,  and,  on  squared  paper,  draw 
a  graph  showing  the  variation  of  sensitiveness  with 
the  load. 

(3)  Clean  a  porcelain  crucible  and   lid.      Weigh   to   o-i 

milligram,  (a)  the  crucible,  (6)  the  lid,  (c)  the 
crucible  and  lid.  Keep  a  record  of  all  the  observa- 
tions from  which  the  zero-point,  the  various  resting- 
points,  and  the  final  results  are  obtained. 

Calibration  of  Weights. 

Since  only  weight  ratios  and  not  absolute  weights  are  of 
importance  in  analytical  chemistry,  it  is  not  essential  that  the 
unit  weight  adopted  should  represent  a  true  or  standard 
gram ;  but  it  is  important  that  the  various  pieces  in  a  set 
should  agree  amongst  themselves,  e.g.,  that  each  of  the 
i -gram  weights  should  be  exactly  100  times  the  weight  of 
each  centigram,  and  one-tenth  that  of  each  lo-gram  piece. 
It  ought  to  be  an  invariable  rule  to  test  a  set  of  weights  before 
it  is  used  for  accurate  work. 

For  ordinary  purposes,  the  method  of  direct  weighing  is 
accurate,  even  if  the  arms  of  the  balance  are  not  exactly 
equal  in  length — provided  the  object  to  be  weighed  is  always 
placed  on  the  same  pan  of  the  balance — but  it  does  not  follow 


CALIBRATION  OF  WEIGHTS  11 

that  two  masses  which  counterpoise  one  another  are  equal  in 
weight ;  for,  evidently,  if  the  lengths  of  the  balance  arms  are 
L  (left)  and  R  (right),  equipoise  with  two  weights  Wx  and 
W9  on  the  left  and  right  pans  respectively  will  be  established 
when  WiL  -  W2R,  and  Wl  =  W2  only  if  L  =  R. 

It  is  impossible,  therefore,  to  compare  the  masses  of,  for 
instance,  two  lo-gram  weights  by  a  single  direct  weighing 
unless  the  arms  of  the  balance  are  equal ;  and,  as  this 
condition  is  seldom  fulfilled,  either  the  ratio  of  the  lengths  of 
the  arms  must  be  found,  or  the  method  of  comparison  must 
be  such  that  the  effect  of  inequality  of  the  arms  is  eliminated. 

The  brass  weights  in  a  set  usually  comprise  the  following 
pieces: — 

50,     20,     10',     10",     5,     2,     i',     i",     i'". 

Before  commencing  the  calibration,  the  various  pieces  of  the 
same  nominal  value  must  be  distinguished  by  means  of  one 
or  more  minute  marks  made  upon  each  with  the  point  of  a 
knife.  The  comparison  of  the  weights  is  made  by  the 
method  of  double  weighing,  which  eliminates  the  error  due  to 
inequality  of  the  arms  of  the  balance  and  provides  data  from 
which,  if  desired,  the  ratio  of  the  lengths  of  the  arms  can  be 
calculated.  The  procedure  is  as  follows  : — 

Find  the  zero-point  of  the  balance.  Place  the  5O-gram 
weight  on  the  left  scale-pan  and  the  remainder  of  the  brass 
weights  on  the  right.  Determine  the  resting-point.  If  it 
differs  from  the  zero-point,  find,  by  the  method  already 
described,  what  additional  weight  must  be  added  to  either 
side  of  the  balance  in  order  to  give  exact  counterpoise.  Now 
interchange  the  weights,  placing  the  5o-gram  weight  on  the 
right,  and  again  determine  the  difference  between  the  loads. 
If,  for  example,  the  two  weighings  were 

Left.  Right. 

50  +  2-0  mg.  =  20+  io'+  io"  +  5  +  2+  i'  +  i"  +  i'", 
and     20+  io'  +  io"+  5  +  2+  i'+  i"+  i'"=  50  +  0-4  mg., 

the  average  of  the  two  weighings  is 

50+  1-2  mg.  =  20+  io'  +  10"  +  5  +  2+  i'+  i"+  i'", 
or  50         =  20+10'  +  io"+5  +  2+  i'+i"+i'"— 1-2  mg. 


12 


GENERAL  PRINCIPLES 


In  the  same  way  compare  the  2O-gram  weight  with  the 
sum  of  10'  and  10",  10'  with  10",  and  10'  with  the  sum  of  5,  2, 
i',  i",  and  i1". 

The  effect  of  inequality  of  the  arms  varies  with  the  load, 
and,  if  it  is  found  at  this  stage  with  10  grams  in  each  pan 
that  the  difference  between  two  weighings  is  not  more  than 
02  milligram,  only  one  weighing  need  be  made  in  compar- 
ing the  smaller  weights.  Proceed  then,  further,  to  compare 
the  5-gram  weight  with  the  sum  of  2,  i',  i",  and  i'";  the  2- 
gram  weight  with  i'  +  i",  and  also  i"  with  i',  and  i'"  with  i'. 

In  column  2  of  the  following  table  the  results  obtained 
with  a  set  of  weights  are  arranged  in  order,  beginning  with 
the  small  weights. 


Calculated  value 

Nominal  value 
of  weights. 

Observed  results  of  double  weighings. 

of  each  piece 
in  terms  of  weight 
marked  1'. 

mg. 

mg 

I' 

=       .               provisional  unit. 

I' 

l" 

--                                   l' 

-0-1 

—         l'      -    0-1 

I'" 

l' 

+  0-1 

=         l'      4-    0-1 

2 

i'+i" 

±0 

=       2  x      -    0-1 

5 

=                             2+l'+l"+l'" 

-C-2 

=      5  x     -  0-3 

10' 

5  +  2  +  I/  +  l"+l'" 

-0-5 

=    10  x     -  0-9 

10" 

10' 

-0-2 

=      IOX       -     !•! 

20 

10'  +  10" 

-0-5 

=      20  X       -     2-5 

50 

=        20+Io'+Io"+5  +  2+l'  +  l"+l'" 

-  1-2 

=    50  x     -   6-1 

Total  loo 

--  50  +  20  +  10',+  10"+  5  +  2  +  I  '  +  l"  +  l'" 

=  IOOX  l'-  II-O 

Now  select,  as  a  provisional  unit,  one  of  the  single  gram 
pieces,  say  i',  and  express  the  value  of  each  of  the  others  in 
terms  of  this  unit.  The  result  of  this  is  shown  in  column  3. 
The  transformation  is  easily  made  if  the  successive  rows  are 
extended  in  order,  beginning  with  the  first.  For  instance, 
take  weight  5,  which  is  equal  (see  column  2)  to  the  sum  of 
the  preceding  weights  minus  0-2  milligram ;  but  (column  3) 
the  sum  of  the  preceding  weights  is  equal  to  5x1'  — o-i 
milligram,  therefore  weight  5  is  equal  to  5x1'  — 0-3 
milligram. 

The  apparent  large  error  in  the  5<>gram  weight  is  in 
reality  the  cumulative  effect  of  the  errors  in  the  smaller 
weights.  In  order  to  get  rid  of  this  additive  effect,  the 


CALIBRATION  OF  WEIGHTS 


13 


assumption  is  now  made  that  the  sum  of  all  the  weights  is 
exactly  100  grams,  and  the  total  error  of  ii-o  milligrams  is 
then  distributed  amongst  the  weights  in  appropriate  propor- 
tion, TV  of  the  error  going  to  each  of  the  lo-gram  weights,  J-  to 
the  2O-gram  weight,  and  so  on. 
The  sum  of  the  weights, 

loox  i'—  u-o  milligrams  =  100  grams, 
or  100  x  i'  =  100  grams  +  i  i-o  milligrams, 
or  i'=  i  gram  +  o-i  i  milligram.  » 

The  value  of  the  provisional  unit,  i',  is  thus  i  gram-f-o-ii 
milligram. 

Now  substitute  this  value  for  each  i'  in  column  3.     The 
following  are  the  results  : — 


Nominal 
value. 

Actual  value. 

Error. 

i' 

=    i  gram  +  0-11  mg. 

=         I-OOOI 

•+o-i  mg. 

i" 

=     i  gram  +  0-11  mg.  -o-i  mg. 

=         I  -0000 

nil. 

i'" 

=    i  gram  -fo-ii  mg.  +  o-i  mg. 

=         I-OOO2 

+  0-2  mg. 

2 

=    2  grams  +  o-22  mg.  -o-i  mg. 

=         2-0001 

+0-1  mg. 

5 

=    5  grams  +  0-5  5  mg.  -  0-3  mg. 

-         5-0003 

+  0-3  mg. 

10' 

=  10  grams  +  i-  1    mg.  -o-gmg. 

=      10-0002 

+  0-2  mg. 

10" 

=  iograms  +  i-i    mg.-i-img. 

=      10-0000 

nil. 

20 

=  20  grams  +  2-2    mg.  -2-5  mg. 

=  19-9997 

-0-3  mg. 

50 

=  50  grams  +  5'5    mg.-6-img. 

=    49-9994 

-0-6  mg. 

Total  100 

=  loo  gms. 

±0-0  mg. 

It  will  be  observed  that  the  sum  of  all  the  errors  is  now 
zero,  a  necessary  consequence  of  the  assumption  that  the  sum 
of  all  the  weights  is  equal  to  100  grams.  The  error  in  the  50- 
gram  piece  is  considerable,  and  the  minus  sign  indicates  that, 
when  the  5o-gram  weight  is  used  in  a  weighing,  0-6  milligram 
should  be  deducted  from  the  observed  weight.  It  may  be 
noted,  however,  that  in  a  "  difference  "  weighing,  if  the  same 
pieces  as  far  as  possible  are  used,  the  actual  error  in  the 
result  may  be  nil,  or  very  small,  even  if  the  corrections  are 
not  applied. 

The  sum  of  all  the  fractions,  viz.,  •5  +  -2  +  -i/+-i"  +  .o5 
+  -02  +  .oi'+.oi"  +  -oi//'(therider),  is  next  compared  with  one 
of  the  single  gram  weights.  It  is  not,  as  a  rule,  considered 
necessary  to  compare  the  fractions  amongst  themselves, 
unless  the  sum  differs  much  from  I  gram. 


14 


GENERAL  PRINCIPLES 


NOTES  ON  GENERAL  APPARATUS. 

The  following  notes  are  intended  to  form  a  guide  in  the 
selection  of  suitable  sizes  and  shapes  of  certain  common  pieces 
of  apparatus.  The  special  apparatus  required  for  volumetric 
and  for  gravimetric  analysis  is  described  in  Parts  II.  and 
III.  respectively. 

Wash-bottle. — A  500  to  700  c.c.  round  flask  is  the  most 
convenient  size.  The  jet,  which  must  deliver  a  fine  stream  of 
water,  should  be  within  easy  reach  of  the  forefinger,  in  order 
that  only  one  hand  may  be  necessary  to  manipulate  the  wash- 
bottle  (Fig.  i).  The  neck  of  the  flask  should  be  covered 


FIG.  i. 


with  a  piece  of  corrugated  paper,  or  thick  string  should  be 
wrapped  round  it,  in  order  to  protect  the  hand  when  hot 
water  is  used. 

Beakers. — Jena  glass  beakers,  provided  with  a  spout, 
are  most  satisfactory.  The  spout  is  not  for  convenience 
in  pouring,  but  provides  an  outlet  for  steam  or  escaping  gas 
when  the  beaker  is  covered  with  a  clock-glass ;  it  prevents 
the  sealing  of  the  beaker  with  a  ring  of  liquid,  portions  of 
which  may  be  projected  during  boiling  and  occasion  loss. 
The  spout  also  forms  a  convenient  place  at  which  a  stirring- 
rod  may  protrude  from  a  covered  beaker. 


NOTES  ON  GENERAL  APPARATUS  15 

The  size  of  a  vessel  must  be  chosen  with  due  regard  to  the 
total  volume  of  liquid  which  it  is  to  contain,  i.e.  neither  too 
large  nor  too  small :  for  precipitations  in 
gravimetric  analysis,  300  c.c.  and  400  c.c. 
beakers  (4}  to  5|-  inches  high)  are  the  most 
generally  useful  sizes,  and  for  ordinary  titra- 
tions  200  c.c.  and  300  c.c.  conical  beakers 

(Fig.  2). 

Flasks. — Jena  glass  conical  flasks  (200  to 
250  c.c.)  with  wide  mouths  (i  in.)  are  con- 
venient for  many  purposes. 

Casseroles. — Porcelain  casseroles  (Fig.  3)  are  used  for 
the  same  purposes  as  porcelain  basins,  and 
are  more  convenient  to  handle. 

Funnels. — The  most  useful  sizes  are  2\ 

inches   and    2f  inches  diameter.    The  sides 

of  funnels  must  be  plain,  and  should  enclose 

an  angle  of  60°.     The  stem  should  be  fairly  long  but  not  too 

wide,  and  very  slightly  constricted  where  it  joins  the  funnel 

cone.     The  end  of  the  stem  is  cut  obliquely. 

Stirring-rods. — Very  light  rods  for  use  in  beakers  may 
be  made  from  glass  tubing,  4-5  mm.  diameter,  by  care- 
fully sealing  both  ends  in  the  blowpipe  flame.  Open  glass 
tubes  must  not  be  used  as  stirring-rods.  If  made  from 
glass  rod,  the  ends  should  be  rounded  in  the  Bunsen  or 
blowpipe  flame.  The  length  of  a  stirring-rod  should  be 
suited  to  the  size  of  the  vessel  in  which  it  is  to  be  used,  e.g. 
(i)  2  to  3  inches  longer  than  the  height  of  a  beaker,  or  (2) 
not  more  than  i  inch  longer  than  the  diameter  of  a  basin.  If 
a  beaker  has  no  spout  and  is  to  be  covered  with  a  clock-glass 
without  removing  the  rod,  a  shorter  rod  that  will  rest 
obliquely  inside  the  beaker  without  touching  the  clock-glass 
must  be  used ;  but  if  the  beaker  has  a  spout,  the  use  of  a 
longer  _rod  is  possible  and  is  more  convenient. 

Desiccators. — Either  concentrated  sulphuric  acid  or  lumps 
of  fused  calcium  chloride  may  be  used  as  the  drying  agent. 
The  layer  of  sulphuric  acid  should  not  be  more  than  \  inch 
deep,  and  if  the  desiccator  is  in  regular  use  the  acid  should 
be  renewed  occasionally.  The  ground  rim  of  the  desiccator 
should  be  greased  with  vaseline,  sparingly  used. 


16  GENERAL  PRINCIPLES 

Wire  Gauze. — Tinned  iron  wire  gauze,  5  inches  square, 
with  an  asbestos  centre,  is  very  durable,  and  forms  a  satis- 
factory and  flat  support  for  a  beaker  or  flask  which  is  to  be 
heated. 

Paper  Mats. — By  placing  beakers  and  flasks  containing 
liquid,  not  directly  on  the  bench  top,  but  on  paper  mats 
(4  inches  diameter)  or  on  pieces  of  thick  blotting  paper,  the 
risk  of  scratching  the  glass  with  sand  grains,  etc.,  often  the 
cause  of  subsequent  fracture,  is  avoided.  Vessels  which  are 
to  be  weighed  should  also  be  placed  on  a  piece  of  clean 
paper  and  not  directly  on  the  bench. 

PREPARATION  OP  THE  SUBSTANCE  FOR  ANALYSIS. 

Pure  Salts. — As  a  rule,  the  so-called  "  puriss  "  or  "  chemi- 
cally pure"  salts  of  commerce  may  be  used  without  special 
purification  for  practising  typical  methods  of  analysis  and  for 
the  preparation  of  standard  solutions. 

In  the  case  of  a  salt  of  doubtful  purity,  a  good  specimen 
can  usually  be  obtained  by  recry stall isation.  About  20 
grams  of  the  salt  are  dissolved  in  the  minimum  quantity  of 
hot  water  contained  in  a  beaker.  The  hot  solution  is  poured 
through  a  fluted  filter  placed  in  a  funnel  with  a  very  short 
stem  (|  inch),  and  the  clear  filtrate  is  received,  with  constant 
stirring,  in  a  porcelain  basin  which  is  cooled  by  placing  it 
in  a  large  dish  containing  cold  water.  The  fine  crystalline 
"  meal "  obtained  in  this  way  is  then  filtered,  a  platinum  cone 
but  no  filter  paper  being  placed  in  the  funnel.  The  salt  is 
well  pressed  down  in  the  funnel  and  the  mother  liquor 
removed  as  far  as  possible  by  means  of  the  filter-pump.  The 
crystals  are  then  pressed  between  filter  paper  and  are  then 
"  air-dried "  for  twelve  hours,  first  by  spreading  upon  filter 
paper,  and  then,  as  paper  is  itself  hygroscopic,  on  a  clock- 
glass,  dust  being  excluded.  Salts  which  effloresce  must  not 
be  exposed  to  the  air  for  very  long,  but  should  be  dried  as 
quickly  as  possible  and  bottled.  Deliquescent  salts  require 
special  treatment.  If  the  salt  contains  no  water  of  hydration, 
it  may  be  dried  in  a  desiccator ;  and  if  it  suffers  no  alteration 
at  100°  or  at  higher  temperatures,  it  may  be  dried  in  the 
steam-oven  or  air-oven. 


CRUSHING  AND  GRINDING  17 

Minerals  and  Rocks. — In  the  first  place,  a  representative 
sample  must  be  obtained.  Minerals  are  often  more  or  less 
contaminated  with  adhering  gangue.  If  the  mineral  is  in 
the  form  of  lumps,  a  number  of  pieces,  as  free  as  possible 
from  earthy  matter,  are  picked  for  the  analysis.  In  the  case 
of  rocks,  a  few  chips  broken  from  a  hand  specimen  will 
usually  represent  the  average  of  the  whole  mass.  The  picked 
sample,  which  should  weigh  about  10  grams,  must  then  be 
powdered. 

If  the  material  is  hard,  it  is  first  broken 
into  coarse  powder  in  a  "percussion"  mortar 
(Fig.  4).  The  mortar  consists  of 'three  pieces — 
a  block  (A),  a  hollow  cylinder  (B),  and  a  pestle 
(C) — all  of  very  hard  steel.  The  selected 
lumps,  one  piece  at  a  time,  are  placed  in  the 
cylinder  (which  fits  into  a  depression  in  the 
block)  and  are  crushed  by  striking  the  pestle 
with  a  hammer.  The  coarsely  powdered  sub-  FlG 
stance  is  emptied  out  on  glazed  paper  and  is 
then  ground  in  an  agate  mortar,  in  very  small  quantities  at 
a  time,  until  every  trace  of  grittiness  has  disappeared. 

In  order  that  the  decomposition  of  the  mineral  by  acids 
or  by  fusion  with  alkali  carbonates,  etc.,  may  be  successfully 
accomplished,  a  very  fine  powder  is  often  essential ;  on  the 
other  hand,  prolonged  powdering  is  not  always  necessary,  and 
may  even  lead  to  error  in  the  analysis.  If  the  mineral 
contains  ferrous  compounds,  for  example,  partial  oxidation 
of  the  iron  may  occur  during  the  grinding  process.  Finely 
ground  powders  may  also  take  up  an  appreciable  amount  of 
water  from  the  air,  and  water  of  hydration  may  be  expelled 
from  minerals  by  long-continued  grinding.  Each  mineral  or 
rock  demands  individual  treatment  in  this  respect,  and  it  is 
impossible  to  give  general  rules.  It  is  advisable,  however,  to 
use  the  coarsest  powder  that  is  likely  to  yield  to  the  subsequent 
treatment. 

Metals  and  Alloys. — In  the  case  of  the  softer  metals, 
pieces  suitable  for  analysis  may  be  cut  from  the  main 
sample  by  means  of  shears  or  a  steel  chisel.  If  this  is  not 
practicable,  a  representative  sample  should  be  obtained  in  the 
form  of  borings  by  means  of  a  steel  drill.  If  the  borings  are 

B 


18 


GENERAL  PRINCIPLES 


contaminated  with  oil,  they  must  be  washed  with  ether  in  a 
Soxhlet  apparatus  and  then  dried. 


Weighing  the  Substance  for  Analysis. 

The  accuracy  required  in  this  operation  depends  on 
the  amount  of  substance  that  is  to  be  weighed.  If  less  than 
i  gram  is  to  be  taken,  the  weight  ought  to  be  accurate  to 
o-i  milligram.  If  several  grams  are  necessary,  weigh  to  the 
nearest  milligram ;  and  if  the  net  amount  is,  say,  10  grams, 
weigh  to  the  nearest  centigram,  i.e.  to  0-005  gram.  The 
weight  of  the  substance  is  always  found  by  "difference," 
and  is  usually  determined  in  one  of  the  following  ways : — 

(1)  Place   2  or   3   grams   of  the   substance,  or    a  larger 

quantity  if  necessary,  in  a  clean  dry  weighing-bottle 
(Fig.  5),  and  weigh  the  bottle  and  its  contents. 
Shake  from  the  bottle  into  the  vessel  in  which  the 
next  operation  is  to  take  place,  a  quantity  which, 
judged  by  the  eye,  is  approximately  equal 
C^  ~~~p  to  that  prescribed,  taking  care  that  none 
of  the  substance  is  lost  in  the  process. 
Weigh  the  bottle  with  the  remaining 
substance  again.  The  difference  between 
the  two  weighings  gives  the  weight  of 
substance  taken.  It  does  not  matter 
FIG.  5.  although  the  weight  is  a  little  more,  or 
a  little  less,  than  that  desired.  If  it  is 
considerably  less,  a  further  quantity  may  be  shaken 
out  and  the  second  weighing  repeated.  If  it  is  con- 
siderably more,  no  attempt  should  be  made  to  return 
part  of  the  substance  to  the  weighing-bottle,  but  the 
whole  operation  should  be  commenced  afresh.  The 
bottle  should  be  handled  as 
little  as  possible  between  the 
weighings. 

(2)  Weigh  a  nickel  scoop.    (A  glass 

or  platinum  scoop,  or  a  watch- 
glass    may   be  used    instead.) 

With  the  forceps  lift  the  scoop  off  the  balance-pan, 
and,  with  a  spatula,  place  upon  it  what  is  judged  to 


p      6 
Scoop  for  Wdghing< 


AMOUNT  OF  SUBSTANCE  FOR  ANALYSIS  19 

be  the  right  quantity  of  the  substance.     Re-weigh  the 
vessel  and  contents. 

(3)  If  the  vessel  in  which  the  substance  is  to  be  dissolved, 
heated,  etc.,  is  comparatively  small  and  light,  such  as  a 
crucible,  weigh  the  substance  directly  in  the  tared 
vessel. 

The  first  method  should  be  used  if  several  portions  of  the 
substance  have  to  be  weighed,  the  separate  quantities  being 
successively  shaken  out  of  the  weighing-bottle,  which  is 
weighed  after  each  operation. 

Liquids  and  volatile  or  hygroscopic  solids  must  be  weighed 
in  a  stoppered  bottle. 

Amount  of  Substance  required  for  Analysis,  and 
Limits  of  Allowable  Error. 

Experimental  errors  are  of  two  kinds:  (i)  more  or  less 
unavoidable  errors,  depending  on  the  method  of  analysis 
employed  ;  and  (2)  accidental  errors,  for  the  most  part  avoid- 
able, arising  from  want  of  care  in  carrying  out  the  work — 
including  the  use  of  unsuitable  or  faulty  apparatus — or  from 
lack  of  manipulative  skill  on  the  part  of  the  worker.  The 
first  class  of  error  can  be  minimised  by  the  choice  of  good 
methods,  whilst  careful  attention  to  every  detail  and  much 
practice  will  help  to  eliminate  errors  of  the  second  class. 

After  the  experimental  errors  have  been  reduced  to  a 
minimum,  the  percentage  error  in  the  final  result  depends 
on  the  amount  of  substance  taken  for  the  analysis.  In 
volumetric  analysis  the  unavoidable  error  depends  mainly 
on  the  precision  with  which  the  amount  of  the  standard  solu- 
tion required  in  the  process  can  be  determined.  The  error 
in  this  measurement  varies  from  o-oi  to  0-05  c.c.  Taking  the 
higher  limit,  and  assuming  that  only  5  c.c.  of  the  standard 
solution  is  required,  the  error  is  equivalent  to  I  per  cent. ; 
but  if  25  c.c.  is  required,  the  same  error  in  the  measure- 
ment represents  a  percentage  error  of  only  0-2.  As  a 
general  rule,  then,  the  amount  of  substance  taken  should 
be  such  that  from  20  to  30  c.c.  of  the  standard  solution  is 
required  for  each  measurement.  The  total  error  in  volu- 


20  GENERAL  PRINCIPLES 

metric  analysis  by  an  accurate  method  should  not  exceed  0-3 
per  cent. 

In  simple  gravimetric  analysis  the  amount  of  substance 
taken  should  be  sufficient  to  give  from  0-2  to  0-5  gram  of  precipi- 
tate in  the  final  weighing.  The  unavoidable  error  arising  in  the 
course  of  the  work  should  not,  in  general,  exceed  i  milligram. 
One  milligram  represents  an  error  of  I  per  cent,  if  the  weight 
of  the  whole  precipitate  is  100  milligrams,  but  only  0-2  per 
cent,  if  the  precipitate  weighs  500  milligrams.  The  amount 
of  substance  taken  should  not,  therefore,  be  too  small.  The 
manipulation,  however,  and  especially  the  filtration  and 
washing  of  a  large  quantity  of  a  bulky,  flocculent  precipitate 
like  ferric  hydroxide,  is  difficult  and  tedious,  and  in  such 
cases  the  minimum  quantity  of  02  gram  should  be  aimed  at ; 
in  other  cases,  like  silver  chloride  or  barium  sulphate,  0-5 
gram,  or  even  i  gram  if  need  be,  is  easily  dealt  with. 

In  complex  analysis,  when  a  large  number  of  constituents 
has  to  be  determined,  no  general  rules  can  be  given  here ;  an 
amount  varying  from  0-5  to  2  grams  is  usually  suitable. 

SOLUTION  OP  THE  SUBSTANCE. 

Provided  the  nature  of  the  substance  and  solvent  allows, 
the  substance  is  brought  into  solution  in  the  same  vessel  in 
which  the  next  operation  is  to  take  place.  As  a  rule,  either 
a  beaker,  a  flask,  or  a  porcelain  basin  is  suitable. 

In  regard  to  the  choice  of  vessels  for  quantitative  analysis, 
it  should  be  remarked  that  the  solvent  action  on  glass  of 
water,  acids,  and  more  especially  alkaline  solutions,  is 
considerable,  and  in  exact  work  it  cannot  be  neglected.  The 
amount  dissolved  depends  on  the  nature  of  the  glass,  and 
increases  with  the  temperature  and  with  the  length  of  time 
the  glass  and  liquid  are  in  contact.  It  is  considerably  less  in 
the  case  of  glass  vessels  that  have  been  in  use  for  some  time. 
Porcelain  and  borosilicate  glass  of  the  Jena  type  resist  the 
action  of  solvents  much  better  than  ordinary  glass,  and  should 
be  used  as  far  as  possible  in  preference  to  the  latter. 

If  precipitation  is  to  follow  solution,  the  weighed  substance 
is  brought  into  solution  in  a  beaker.  Solution  may  be 
promoted,  if  necessary,  by  heating  the  beaker  (supported  on 


SOLUTION  AND  EVAPORATION 


21 


wire  gauze)  with  a  Bunsen  flame,  or  by  warming  on  a  steam- 
bath.  If  actual  boiling  is  required,  or  if  gases  are  evolved, 
loss  of  substance  from  spirting  or  spray 
is  prevented  by  covering  the  beaker  with 
a  clock-glass  (Fig.  7).  The  clock-glass 
should  be,  at  most,  half  an  inch  larger 
than  the  mouth  of  the  beaker,  and,  in 
order  to  provide  an  outlet  for  steam  or 
escaping  gas,  the  beaker  should  have  a 
spout.  If  evaporation  is  to  follow  solution, 
a  porcelain  basin  is  used,  also  covered  with 
a  clock-glass  of  suitable  size.  In  the  case 
of  a  flask,  loss  of  substance  is  prevented 
by  placing  a  small  funnel  in  the  mouth 
of  the  flask  (Fig.  8),  or  by  clamping  the  flask  in  a  sloping 
position.  A  flask  should  be  used,  as  a  rule,  if  prolonged 
heating  with  volatile  acids  is  necessary,  and  in  this 
case  a  glass  bulb  may  be  placed  in  the  mouth  of  the  flask 
(Fig-  9). 


FIG.  7. 


FIG.  8. 


FIG.  9. 


After  solution  is  complete,  or  decomposition  ended,  the 
cover  of  the  vessel  must  always  be  rinsed  with  a  jet  of  water 
from  the  wash-bottle,  and  the  washings  added  to  the  solution. 


EVAPORATION. 

In  this  operation  three  points  demand  special  attention, 
viz. : — 

1.  No  loss  of  substance  must  occur  in  the  process. 

2.  It  should  take  place  as  rapidly  as  possible — with  due 

regard  to  point  I. 


GENERAL  PRINCIPLES 

3.  Contamination  from  without  must  be  guarded  against. 
Loss  of  substance  is  prevented  by  evaporating  on  the 
steam-bath,  thus  avoiding  actual  ebullition  of  the 
liquid.  As  a  rule  the  process  should  be  conducted  in 
a  porcelain  basin,  not  more  than  two-thirds  rilled  ;  in 

a  beaker  evaporation  is 
slow.  Dust,  etc.,  is  ex- 
cluded by  placing  over  the 
basin  a  clock  -  glass,  of 
larger  diameter  than  the 
basin,  supported,  convex 
side  upwards,  on  a  glass 
tripod  (Fig.  10).  The  latter 
is  made  from  thin  glass 
rod,  first  bending  it  to 

form  a  triangle  with  sides  of  5  to  6  inches,  and  then 
attaching  legs  about  ij  inches  long  at  the  corners. 
(The  size  depends  on  the  diameter  of  the  basin.) 
The  rate  at  which  evaporation  proceeds  depends  on  the 
continuous  removal   of  the  vapour   over  the  surface  of  the 
liquid  by  means  of  a  current  of  air,  and  the  operation  should 
therefore  be  conducted  in  a  good  draught. 

Evaporation  at  the  boiling  point  may  be  conducted  in  a 
flask,  supported  obliquely  and  only  half  filled.  This  method 
is  also  useful  if  effervescence  due  to  the  escape  of  gas  occurs 
on  heating. 

PRECIPITATION. 

This  is  generally  conducted  in  beakers.  Conical  flasks 
are  sometimes  preferable,  but  round  flasks  are  unsuitable. 
Jena  glass  or  porcelain  vessels  should  be  used  in  preference 
to  ordinary  glass  for  precipitations  with  alkaline  hydroxides 
and  carbonates  and  with  ammonia.  The  following  general 
considerations  regarding  precipitation  in  quantitative  analysis 
may  be  noted  : — 

(i)  The  precipitation  must  be  practically  complete.  In 
order  to  secure  this,  it  is  usually  necessary  to  adhere 
more  or  less  rigidly  to  certain  prescribed  conditions, 
such  as,  for  example,  the  amount  of  acid  present  in 


PRECIPITATION  AND  FILTRATION  23 

the  solution.  In  the  case  of  crystalline  precipitates 
more  especially,  a  certain  interval  of  time  must 
elapse  before  the  precipitation  may  be  regarded  as 
complete,  and  the  filtration  must  be  postponed  for 
one,  two,  or  even  for  twelve  hours. 

(2)  The  precipitate  must  be  free  from  contamination  with 

other  substances.  In  spite  of  all  precautions  to 
prevent  it,  partial  precipitation  of  one  substance  with 
another  sometimes  occurs.  In  such  cases  it  is  often 
possible  to  effect  a  more  or  less  complete  separation 
by  redissolving  the  precipitate,  after  filtration,  and 
precipitating  a  second  time. 

(3)  The  precipitate  must   be  of  known   composition,   or 

must  be  capable  of  easy  conversion  into  a  substance 
of  known  composition. 

(4)  As  a  rule,  a  slight  excess  of  the  reagent  must  be 

added ;  a  large  excess  is,  however,  generally  pre- 
judicial, and  is  a  common  source  of  error.  The 
reagent  should  be  added  carefully,  a  little  at  a  time, 
until,  after  allowing  the  precipitate  to  settle,  it  is  seen 
that  another  drop  produces  no  further  precipitation. 

(5)  It   is  important  to  note   the  exact  conditions  under 

which  certain  precipitates  are  obtained  in  a  granular 
or  crystalline  form,  instead  of  being  so  finely  divided 
that  they  pass  through  the  filter.  If,  for  example, 
barium  sulphate  is  precipitated  in  the  cold  from 
concentrated  solution,  it  is  practically  impossible  to 
filter  it.  A  granular  precipitate  can  be  obtained, 
however,  by  adhering  to  the  following  conditions : — 
(a)  the  solution  must  be  dilute ;  (b)  it  must  contain 
a  little  hydrochloric  acid  ;  (c)  it  must  be  heated  to  the 
boiling  point ;  (d)  the  reagent,  barium  chloride,  must 
also  be  hot  and  dilute,  and  must  be  added  slowly. 

FILTRATION. 

In  quantitative  analysis,  filtration  must  be  conducted  with 
much  greater  care  than  is  sometimes  given  to  the  operation 


24  GENERAL  PRINCIPLES 

in   qualitative   analysis.     The   more  important   rules   to   be 
observed  are  the  following  : — 

1.  The  size  of  the  filter  depends,  not  on  the  volume  of  the 

liquid,  but  on  the  bulk  of  the  precipitate  to  be 
separated.  The  precipitate  must  not  more  than  half- 
fill  the  filter.  As  a  rule,  a  filter  paper  9  cm.  in 
diameter  is  large  enough,  but  for  bulky  precipitates 
an  1 1  cm.  paper  is  often  required.1 

2.  The  filter  paper  when  folded  must  be  somewhat  smaller 

than  the  funnel,  e.g.,  a  9  cm.  paper  requires  a  funnel 
5-5  cm.  in  diameter. 

3.  It  is  most  important  that  the  folded  filter  paper  should 

fit  the  funnel  exactly.  If  the  funnel  angle  is  greater 
or  less  than  60°,  the  paper  must  be  folded  with  a 
certain  amount  of  overlap  at  the  second  fold  so  that 
one  cone  is  larger  than  the  other.  In  this  way  an 
accurate  fit  can  be  obtained.  The  filter  is  then  held 
in  place  in  the  funnel,  and,  after  wetting  with  water, 
is  well  pressed  into  contact  with  the  funnel  wall, 
especially  round  the  top.  This  will  prevent  the 
entrance  of  air,  and  if  the  stem  of  the  funnel  once 
fills  with  liquid  it  will  remain  full,  and  the  slight 
suction  will  effect  more  rapid  filtration. 

4.  If  possible,  liquids  should  be  filtered  hot. 

5.  The  under  side  of  the  rim  of  the  beaker  containing  the 

precipitate  should  be  rubbed  at  one  place  (opposite 
the  spout)  with  an  almost  invisible  trace  of  melted 
rubber,  and  at  this  place  the  liquid  should  be  poured 
down  the  stirring-rod  into  the  filter,  directing  the 
liquid  against  the  side  of  the  filter  and  not  into  the 
apex  (Plate  I,  Fig.  i).  The  filter  must  not  be  filled 
quite  to  the  brim. 

1  Schleicher  and  Schiill's  filter  papers,  Nos.  589  and  590,  are 
excellent.  No.  589  is  made  in  three  varieties,  distinguished  as  black, 
white,  and  blue  ribbon.  The  "  black  ribbon  "  paper  filters  very  quickly, 
and  is  suitable  for  flocculent  or  slimy  precipitates  like  Fe(OH)3,  or 
A1(OH)3;  the  "white  ribbon"  (the  most  generally  useful  variety)  is  of 
closer  texture,  and  will  retain  fine  precipitates  like  BaSO4 ;  the  "  blue 
ribbon"  paper  filters  slowly,  and  should  be  used  only  in  conjunction 
with  the  filter-pump. 


PLATE  I. 


FlG.  I. — Washing  by  Decantation. 


FlG.  2. — Transferring  the  Precipitate  to  the  Filter. 

[Face  page  24. 


WASHING  OF  PRECIPITATES  25 

6.  In  order  to  prevent  loss  by  splashing,  the  stem  of  the 
funnel  must  rest  against  the  side  of  the  receiving 
vessel. 

Washing  of  Precipitates. — The  separation  from  a  pre- 
cipitate of  the  soluble  substances  present,  which  filtration 
roughly  effects,  is  completed  by  repeatedly  "washing"  the 
precipitate — usually  with  water.  In  order  to  accomplish 
this  rapidly  and  with  the  minimum  quantity  of  wash- 
water,  the  following  more  or  less  general  rules  should  be 
observed : — 

Before  commencing  to  filter,  allow  the  precipitate  to 
settle ;  then,  without  disturbing  the  precipitate,  pour  as 
much  as  possible  of  the  clear  liquid  into  the  filter. 

If  the  precipitate  is  bulky  or  gelatinous,  like  ferric  or 
aluminium  hydroxide,  time  is  saved  and  less  washing  water 
required  by  using  the  filter-pump. 

Wash  with  hot  water,  if  there  is  no  objection  to  its 
use. 

If  the  precipitate  settles  rapidly,  wash  it  several  times  by 
"  decantation,"  as  follows  : — After  the  supernatant  liquid  has 
been  poured  through  the  filter,  mix  the  precipitate  with 
50  to  80  c.c.  of  water,  allow  it  to  settle  again,  and  once  more 
decant  the  clear  liquid  into  the  filter ;  repeat  the  process  two 
or  three  times.  Washing  by  decantation  gives  rise  to  a 
bulky  filtrate  and  may  often  be  omitted,  especially  if  the 
filter-pump  is  used,  or  if  the  precipitate  is  very  small. 

Transfer  the  precipitate  to  the  filter  by  means  of  a  jet 
of  water  from  the  wash-bottle  in  the  manner  shown  in 
Plate  I,  Fig.  2.  Remove  any  precipitate  adhering  to  the 
sides  of  the  beaker  by  i^eans  of  a  stirring-rod  tipped  with  a 
piece  of  black  rubber  tubing  £  inch  long.  In  place  of  the 
glass  rod  with  a  rubber  tip,  a  trimmed  feather  may  be  used. 
The  plumules  of  the  feather  are  torn  away  to  within  2  cms. 
of  the  end,  and  those  remaining  are  cut  parallel  to  the  quill 
at  a  distance  of  not  more  than  5  mm.  from  it. 

If  any  cracks  or  channels  form  in  a  bulky  precipitate 
(the  result  of  continuing  suction  after  all  the  liquid  has 
passed  through),  close  them  carefully  with  a  jet  of  water,  or 
a  glass  rod.  Be  careful  not  to  use  so  strong  a  jet  of  water, 


26  GENERAL  PRINCIPLES 

or  to  direct  it  in  such  a  way,  that  portions  of  the  precipitate 
are  projected  out  of  the  filter. 

Allow  each  washing  to  pass  through  the  filter  before  the 
next  is  applied.  Wash  the  margin  of  the  filter  paper 
carefully. 

Continue  the  washing  until  the  soluble  substance  can  no 
longer  be  detected  in  the  filtrate.  Avoid  over-washing, 
as  no  precipitate  is  quite  insoluble. 

Towards  the  end  of  the  process  endeavour  to  collect  the 
precipitate  as  far  as  possible  in  the  apex  of  the  filter. 

Never  put  anything  but  distilled  water 
in  the  wash-bottle.  Separate  small  wash- 
bottles  (300  c.c.)  should  be  used  for  am- 
monia, hydrogen  sulphide,  alcohol,  etc. 
In  order  to  prevent  the  back-flow  of 
ammonia,  etc.,  to  the  mouth,  a  valve  may 
be  used.  To  make  the  valve,  a  slit 
(f  inch)  is  cleanly  cut  in  a  piece  of  narrow 
rubber  tubing  (i|-  inch).  One  end  of  the 
FlG  II  rubber  tube  is  closed  with  a  plug  of  glass 

rod,   and   the  valve   is    then   attached   to 
the  blow-tube  inside  the  wash-bottle  (Fig.  n). 

Use  of  the  Filter-pump. — Accelerated  filtration,  by  means 
of  the  filter-pump,  is  frequently  advantageous,  especially  in 
the  case  of  bulky,  gelatinous,  or  slimy  precipitates  like 
aluminium  or  chromic  hydroxides,  or  zinc  sulphide.  The 
platinum  cone  which  is  used  to  support  the  filter  paper  must 
be  well  made  and  in  good  condition  ;  a  bad  cone  with  rough 
edges  is  often  itself  the  cause  of  rupture  of  the  filter  paper. 
In  place  of  a  platinum  cone,  one  of  toughened  paper  may  be 
used.1  Gentle  suction  only  should  Jpe  used,2  and,  unless  the 
filtration  is  continuous,  the  suction  should  be  interrupted  as 
soon  as  all  the  liquid  has  passed  through.  To  effect  this 
most  simply  without  stopping  the  pump,  the  latter  is 
connected  to  the  filter-flask  through  a  T-piece,  one  limb  of 

1  Schleicher  and  Schiill  supply  papers  (No.  574)  of  a  special  shape 
which  admit  of  simple  folding  to  produce   a  cone.     The  smallest  size 
should  be  used. 

2  A  pressure   regulator  for   use    with   the  filter-pump   is   described 
on  p.  114. 


USE  OF  THE  FILTER-PUMP  27 

which  is  closed  by  a  piece  of  rubber  tubing  and  clip  (Fig.  12). 
When  necessary,  the  clip  is  opened  to  admit  air. 
Instead  of  the  filter-pump  it  is  often  more 
convenient   to   use   a   funnel   provided   with   a 
looped  suction  tube  about  20  cm.  (8  inch)  long 


FIG.  12. 

and  4  to  5  mm.  internal  diameter.  The  funnel 
stem  is  cut  short,  and  the  suction  tube  fused 
on,  as  shown  in  Fig.  13. 

In  using  either  the  filter-pump  or  a  suction  tube,  it  is 
most  important  that  the  filter  paper  should  fit  the  funnel 
exactly,  and  that  no  air  leaks  through  along  the  folds  of  the 
paper  at  either  side.  If  air-leakage  sets  in,  the  edge  of  the 
paper  should  be  firmly  pressed  into  contact  with  the  funnel  by 
means  of  the  stirring-rod.  Once  suction  has  been  established 
with  a  suction  tube,  the  filter  should  not  be  allowed  to  empty 
until  all  the  liquid  has  been  filtered. 


PART     II 

VOLUMETRIC  ANALYSIS 

THE  volumetric  analysis  of  a  substance  involves : 

(1)  The  preparation  of  one  or  more  solutions  of  accurately 

known  concentration ; 

(2)  The  use   of  instruments  with  which  the   volumes  of 

the   solutions   used   can   be   quickly  and   accurately 
determined ; 

(3)  Some   means  of  recognising  the  completion  of  the 

chemical  change  which  takes  place. 

The  following  example,  in  which  minor  experimental 
details  are  omitted,  may  be  given  as  an  illustration  of  the 
principles  of  volumetric  analysis.  Suppose  that  it  is  required 
to  determine  the  percentage  of  chloride  in  a  given  substance. 
A  solution  of  silver  nitrate  of  known  concentration  is  prepared 
by  dissolving  a  weighed  quantity  of  pure  silver  nitrate  in  water 
and  making  up  to  a  definite  volume.  A  weighed  quantity  of 
the  substance  to  be  analysed  is  dissolved  in  water,  some  nitric 
acid  is  added,  and  the  silver  nitrate  solution  is  run  in,  in 
small  portions  at  a  time.  When  all  the  chloride  has  been 
converted  into  silver  chloride,  the  next  addition  of  silver 
nitrate  will  yield  no  further  precipitation,  and  it  is,  therefore, 
possible  to  determine  accurately  the  volume  of  silver  nitrate 
solution  necessary  to  precipitate  all  the  chloride.  Since  the 
concentration  of  the  silver  nitrate  solution  is  known,  we  can 
thus  obtain  the  weight  of  silver  nitrate  required.  Each 
AgNO3  will  precipitate  one  equivalent  of  the  chloride,  and 
therefore  the  weight  of  the  chloride  can  be  readily  calculated 
once  the  weight  of  silver  nitrate  equivalent  to  it  is  known. 


STANDARD  SOLUTIONS  29 

Standard  Solutions. — A  solution  of  known  concentration 
for  use  in  volumetric  analysis  is  called  a  standard  solution. 
Standard  solutions  may  sometimes  be  prepared  by  dissolving 
an  accurately  weighed  portion  of  the  substance  in  water  and 
making  the  solution  up  to  a  definite  volume ;  e.g.,  standard 
solutions  of  silver  nitrate  and  sodium  carbonate  may  be 
prepared  in  this  way.  Often,  however,  the  material  to  be 
used  contains  an  unknown  amount  of  impurity  or  of  water 
of  crystallisation,  or  it  may  be  found  unsuitable  for  weighing 
on  account  of  its  deliquescent  or  efflorescent  nature.  In 
such  cases  a  solution  of  approximately  the  required  concen- 
tration is  prepared,  and  the  exact  concentration  is  then  found 
by  titration  against  some  suitable  substance.  For  example, 
concentrated  sulphuric  acid  is  only  approximately  100  per 
cent,  sulphuric  acid,  but  a  standard  solution  may  be  prepared 
by  first  making  a  solution  of  approximately  the  desired 
concentration  on  the  assumption  that  the  concentrated  acid 
is  pure  H2SO4.  The  true  concentration  is  then  found  by 
titration  against  accurately  weighed  quantities  of  pure 
anhydrous  sodium  carbonate. 

Normal  Solutions. — A  standard  solution  may  be  of  any 
concentration,  but  for  convenience  in  calculation  it  is 
advantageous  to  prepare  solutions  which  contain  one  gram- 
equivalent  (or  some  simple  fraction)  per  litre.  A  solution 
which  contains  one  gram-equivalent  of  the  reacting  substance 
per  litre  of  solution  is  called  a  normal  solution. 

The  gram-equivalent  is  the  weight  of  the  substance  in 
grams  which  contains  I  gram  of  replaceable  hydrogen,  or 
which  is  chemically  equivalent  to  it. 

Normal  hydrochloric  acid  therefore  contains  36-47  grams 
HC1  per  litre ;  normal  sulphuric  acid  contains  49-04  grams 
H2SO4 ;  and  normal  sodium  hydroxide  contains  40-01  grams 
NaOH  per  litre.  A  normal  solution  of  silver  nitrate  contains 
on  the  same  principle  169-9  grams  AgNO3  per  litre.  The 
symbol  "  N  "  is  often  employed  as  a  contraction  for  "  normal." 

Volumetric  analyses  of  a  very  important  class  are  known 
as  oxidation  or  reduction  processes,  on  account  of  the  nature 
of  the  chemical  action  involved.  A  normal  solution  of  an 
oxidising  substance  is  one  which  contains  8  grams  of  avail- 
able oxygen  per  litre,  i.e.,  the  weight  necessary  to  oxidise 


30  VOLUMETRIC  ANALYSIS 

i  gram  of  hydrogen.  A  normal  solution  of  potassium  per- 
manganate contains  31-61  grams  KMnO4  per  litre.  Titrations 
with  potassium  permanganate  are  always  carried  out  in  acid 
solutions,  and  the  reaction  which  occurs  is 


2KMnO4  +  4H2SO4  =  2KHSO4  +  2MnSO4  +  3H,O  +  50. 

(The  oxygen  is  not  liberated,  but  goes  to  oxidise  the 
substance  under  analysis.)  It  is  evident  from  the  equation 
that  2  molecules  of  KMnO4  yield  10  equivalents  (or  5 
atoms)  of  oxygen.  A  normal  solution  of  KMnO4  contains, 
therefore,  one-fifth  of  the  gram-molecular  weight  per  litre, 

i.e.    ^  '  3  _  ^  j  .5  1  grams. 

Factors.  —  When  a  solution  is  not  exactly  normal,  the 
concentration  should  be  expressed  in  terms  of  a  normal 
solution,  e.g.,  1-008  N  HC1  and  0-1013  N  NaOH. 

Indicators.  —  In  many  cases  the  completion  of  the  titration 
can  be  made  evident  by  the  addition  of  a  third  substance, 
called  an  indicator.  For  instance,  potassium  chromate  may 
be  used  as  an  indicator  in  the  titration  of  a  chloride  by 
silver  nitrate  (if  the  solutions  are  neutral).  Silver  nitrate 
yields  with  a  chromate  a  bright  red  precipitate  of  silver 
chromate.  So  long  as  any  chloride  is  present,  there  is  no 
permanent  precipitation  of  silver  chromate  —  all  the  silver 
uniting  with  the  chloride  in  preference  to  the  chromate. 
When  all  the  chloride  is  precipitated,  the  next  addition  of 
silver  nitrate  produces  a  red  colour  on  account  of  the  forma- 
tion of  some  silver  chromate. 

The  indicators  used  to  indicate  neutrality  form  a  very 
important  class,  By  their  aid  it  is  possible  to  determine 
the  concentration  of  an  acid  by  titration  with  a  standard 
alkali,  or  vice  versa. 

THE  MEASUREMENT  OP  VOLUMES  OP  LIQUIDS. 

For  the  measurement  of  the  volume  of  a  liquid,  various 
graduated  glass  instruments  are  used,  the  most  important 
being  pipettes,  burettes,  and  graduated  flasks.  Measuring 
cylinders  are  used  for  rough  measurements  only. 

Flasks.  —  Flasks  are  used  in  volumetric  analysis  mainly 


UNIT  OF  VOLUME  31 

for  the  measurement  of  comparatively  large  volumes,  i.e.,  for 
volumes  of  100  c.c.  and  upwards.  A  flask  is  graduated  to 
contain  a  definite  volume  of  liquid. 

Pipettes. — A  pipette  is  used  to  deliver  a  specified  volume 
of  liquid.  Pipettes  are  made  in  various  sizes,  from  i  c.c.  up 
to  100  c.c.  The  10  c.c.  and  25  c.c.  are  the  most  generally 
useful  sizes. 

Burettes. — A  burette  is  used  to  deliver  measured 
quantities  of  a  liquid.  The  most  convenient  size  is  one 
which  delivers  any  volume  up  to  50  c.c.,  with  graduations  at 
every  tenth  of  a  cubic  centimetre. 

The  Unit  of  Volume. 

All  instruments  used  in  volumetric  analysis  are  graduated 
in  c.c.  (cubic  centimetres).  The  true  cubic  centimetre  is  the 
volume  of  I  gram  of  water,  weighed  in  a  vacuum,  at  4°. 
Many  instruments,  however,  are  graduated  in  terms  of 
another  unit,  incorrectly  called  a  cubic  centimetre,  which  is 
the  volume  of  i  gram  of  water  at  15-5°  when  weighed  in  air 
with  brass  weights.1  This  unit  is  02  per  cent,  larger  than 
the  true  cubic  centimetre.  Since  in  volumetric  analysis  only 
relative  volumes  are  required,  it  is  immaterial  which  unit  is 
adopted,  provided  it  is  used  systematically  for  all  instruments. 
A  warning  as  to  the  use  of  both  units  seems  particularly 
necessary,  since  pipettes  and  burettes  are  now  usually 
graduated  in  terms  of  the  true  c.c.,  whilst  the  other  unit 
seems  to  be  more  commonly  used  for  flasks.  In  the  follow- 
ing pages  the  true  c.c.  only  is  used. 

The  average  temperature  of  most  laboratories  is  about 
15°,  and  the  calibration  of  the  measuring  instruments  should 
therefore  be  made  with  water  at  about  this  temperature.  If 
it  is  desired  to  calibrate  a  vessel  at,  for  instance,  15°,  one 
requires  to  know, 

(1)  the  weight  of  water  which  will  occupy  i  c.c.  at  the 

given  temperature,  since  the  density  of  water  varies 
with  the  temperature ;  and 

(2)  the  corrections   to  be  applied  for  the  weight   of  air 

displaced   by  the  water   and   by  the  brass  weights 
respectively. 
1  Temperatures  of  15°  (Dittmar)  and  17-5°  (Mohr)  are  also  adopted. 


32 


VOLUMETRIC  ANALYSIS 


In  the  following  table  the  corrections  for  these  factors  have 
been  introduced. 

Ratio  of  Weight  to   Volume  of  Water,  weighed  in  Air 
with  Brass   Weights. 


Tempera- 
ture. 

Weight  of 
1  c.c. 
in  grams. 

Volume  in  c.c. 
occupied  by 
1  gram. 

Tempera- 
ture. 

Weight  of 
1  c.c. 
in  grams. 

Volume  in  c.c. 
occupied  by 
1  gram. 

10° 

0-9986 

I-OOI4 

1  6° 

0-9979 

I-002I 

11° 

85 

15 

17° 

77 

23 

12° 

84 

16 

1  8° 

76 

24 

13° 

83 

17 

19° 

74 

26 

14° 

82 

18 

20° 

72 

28 

15° 

81 

19 

21° 

70 

30 

Measuring  instruments  as  bought  are  often  inaccurate. 
The  experimental  determination  of  the  errors  in  the 
graduation  of  the  instrument  is  called  calibration.  If  the 
graduations  are  altered  or  adjusted  so  as  to  make  the 
instrument  exact,  it  is  said  to  be  standardised.  Standardised 
instruments,  i.e.,  instruments  of  a  high  degree  of  accuracy, 
are  now  obtainable  commercially.  Such  instruments  are 
more  expensive  than  the  ordinary  instruments,  and  are 
only  accurate  when  used  in  exactly  the  same  way  as  was 
adopted  in  the  standardisation.  It  is  advisable,  therefore, 
that  each  worker  should  standardise  or  calibrate  his  own 
instruments. 


STANDARDISATION  OP  A  FLASK. 

The  flask  should  be  provided  with  a  well-fitting  ground- 
in  glass  stopper,  and  should  have  a  long  narrow  neck.  The 
graduation  mark  should  be  on  the  lower  half  of  the  neck 
(Fig.  14).  The  diameter  of  the  neck  should  not  exceed 
2  cm.  for  a  I  litre  flask,  1-7  cm.  for  a  500  c.c.  flask,  or 
8  mm.  for  a  100  c.c.  flask. 

Flasks  are  graduated  to  contain  (not  deliver)  definite 
volumes,  though  for  special  purposes  a  flask  may  be  so 
graduated  that  it  will  deliver  a  measured  volume  of 
liquid. 


STANDARDISATION  OF  A  FLASK 


33 


Clean  the   flask   and   its  stopper  thoroughly,1  dry   in    a 
steam-oven,  and  weigh  it  after  cooling.     Fill  the  flask  to  the 
graduation  mark  with  distilled  water  which 
is  at  or  very  near  the  room  temperature. 

Examination  of  the  surface  of  the  water 
in  the  neck  of  the  flask  will  show  that  it 
is  not  flat  but  curved,  and  the  level  of 
the  water  must  be  adjusted  so  that  the 
lowest  point  of  the  curved  surface  (the 
meniscus)  coincides  exactly  with  the 
graduation  mark.  Error  due  to  parallax 
is  avoided  when  the  front  and  back  of  the 
graduation  mark  are  seen  as  a  single 
line.  The  meniscus  will  be  clearly  visible 
if  a  white  card  is  held  behind  it.  There 
should  not  be  any  drops  adhering  to  the 
glass  above  the  graduation  mark,  but  this 
will  only  happen  if  the  flask  is  greasy. 

Weigh  the  flask  filled  with  water,  and, 
from  the  weight  of  the  water,  calculate  the 
volume.     The  weight  of  water  should  be 
known  to   i    in   2000,  and  for   larger   sized  flasks   a   rough 
balance,  capable  of  weighing  to  ^  gram,  may  be  used. 

1  Methods  of  Cleaning  Glass  Apparatus.— If  a  pipette  (or  burette)  is 
dirty,  drops  of  liquid  adhere  to  the  walls  and  a  considerable  error  may 
be  introduced.  Several  methods  of  cleaning  may  be  used,  the  choice 
being  largely  a  matter  of  personal  preference. 

1.  Wash  with  sodium  hydroxide  (to  remove  grease),  then  successively 

with  water,  dilute  nitric  acid,  and  finally  several  times  with  water. 

2.  If  the  glass   is  very  greasy,   alcoholic   sodium  hydroxide   or  a 

mixture  of  bench  sodium  hydroxide  and  alcohol  will  be  found 
more  efficient  than  the  aqueous  solution. 

3.  Soap  and  water,  applied  with  a  cloth  or  sponge  tied  to  the  end  of 

a  thin  wooden  rod,  provide  about  the  best  method  for  cleaning 
a  burette. 

4.  Grease  may  be  removed  by  prolonged  treatment  with  chromic 

acid  solution  (a  mixture  of  potassium  dichromate  solution  and 
concentrated  sulphuric  acid).  This  is  slow  but  efficient.  It  is 
best  to  fill  the  vessel  and  allow  it  to  stand  overnight,  or  as  long 
as  possible.  A  pipette  may  be  kept  full  by  filling  almost  to  the 
top  and  closing  at  once  with  a  cap  made  from  an  inch  of  rubber 
tubing  and  a  short  piece  of  glass  rod. 

A  pipette  or  burette  is  clean  if  there  is  no  formation  of  drops  on  the 
surface  of  the  glass  after  the  liquid  is  run  out. 


FIG.  14. 


34 


VOLUMETRIC  ANALYSIS 


Example. — The   following   figures   were  obtained  with  a 
500  c.c.  flask  : — 

Flask  +  water          =   566-8  grams 
Tare  of  flask  =     65-2      „ 

Weight  of  water     =    501-6      „ 

The  temperature  of  the  water  was  found  to  be  12°  (by  plac- 
ing a  thermometer  in  the  water  after  the  weighing  was 
completed).  From  table  on  p.  32  we  find  that  the  volume 
of  i  gram  of  water  at  12°,  weighed  in  air  with  brass  weights, 
is  1-0016  c.c.  The  flask  therefore  contained  (501-6x1-0016) 
=  502-4  c.c. 

The  flask  may  be  used  with  application  of  the  necessary 
correction,  but  it  is  more  convenient  to  make  a  new  gradua- 
tion at  the  correct  place.  Gum  a  piece  of  paper  on  the  neck, 
and  make  a  mark  upon  it  at  what  is 
thought  to  be  the  correct  position.  An 
estimate  of  how  far  this  should  be  from 
the  original  graduation  may  be  obtained 
by  running  in  a  measured  volume  of 
water  from  a  burette  after  the  flask  has 
been  filled  to  the  mark.  Check  the  new 
graduation  as  before,  and  repeat  the 
process,  if  necessary,  until  the  gradua- 
tion is  correct  to  i  in  2000,  z>.,  for  a 
i  litre  flask,  until  the  volume  is  between 
999-5  and  1000-5  c.c. 

Etching  a  line  on  glass. — A  new 
line  should  then  be  etched  on  the  glass 
at  the  correct  position  by  means  of 
hydrofluoric  acid  in  the  following 
manner : — Gum  on  two  strips  of  paper 
completely  round  the  tube,  leaving  only 
a  narrow  space  between  them  where  the  line  is  to  be  etched 
(Fig.  15).  Warm  cautiously  above  a  flame,  and  rub  with  a 
piece  of  paraffin  wax  until  the  paper  is  saturated  with  the 
melted  wax.  When  it  has  solidified,  remove  the  wax  from 
the  line  between  the  two  papers  by  means  of  a  metal  point. 
Fix  a  narrow  strip  of  filter  paper  round  this  line,  and  wet 


FIG.  15. 


USE  OF  A  BURETTE  35 

the  paper  with  hydrofluoric  acid  solution.  (Caution. — Care 
must  be  taken  that  the  hydrofluoric  acid  does  not  come 
into  contact  with  the  skin,  as  it  causes  later  a  particularly 
painful  sore.) 

After  about  ten  minutes,  wash  off  the  hydrofluoric  acid 
and  remove  a  small  portion  of  the  paper  and  wax  to  ascertain 
if  the  glass  is  sufficiently  etched.  If  the  etching  is  insufficient, 
re-wax  and  repeat  the  process,  but  allow  more  time  for  the 
etching  action.  The  glass  is  not  etched  where  it  is  <==* 
protected  by  the  paraffin  wax. 

USE  OF  A  BURETTE. 

For  most  purposes  a  convenient  size  is  one  which 
will  deliver  50  c.c.,  with  graduations  at  each  ^  c.c. 
It  is  fitted  at  the  lower  end  with  a  glass  tap  or  with 
a  rubber  tube  and  clip,  so  that  the  flow  of  liquid 
may  be  regulated  and  stopped  as  desired  (Fig.  16). 
A  glass  tap  must  be  used  with  potassium  per- 
manganate and  iodine  solutions,  since  these  attack 
rubber. 

In  order  that  the  measurements  with  a  burette 
may  be  accurate,  attention  must  be  paid  to  the 
following  points : — 

(1)  The  burette  must  be  clean. 

(2)  The  amount  of  lubricant1  used  must  be  not 

more  than  is  sufficient  to  allow  the  tap  to 
turn  easily. 

(3)  Sufficient  time  must  be  allowed  for  draining 

before  the  reading  is  taken.  To  prevent  any 
error  on  this  account,  it  is  advisable  to  have 
a  fine  jet  so  that  the  liquid  will  not  run  out 
at  more  than  I  c.c.  per  two  seconds.  If  the  FIG.  16. 

J  For  most  purposes,  resin  cerate  or  a  mixture  of  resin  cerate  and 
vaseline  will  be  found  satisfactory. 

A  very  good  lubricant  may  be  prepared  as  follows  :— Heat  together 
on  a  sand-bath  a  mixture  of  6  parts  of  best  soft  rubber  (free  from  in- 
organic filling  material),  3  parts  of  vaseline  and  I  part  of  paraffin  wax. 
Stir  thoroughly  until  the  rubber  has  completely  dissolved. 


36 


VOLUMETRIC  ANALYSIS 


FIG.  17. 


burette  discharges  more  quickly,  the  opening  may 
be  constricted  by  cautious  heating  in  a  flame. 

(4)  The  reading  must  always  be  taken  of  the  graduation 

opposite  the  lowest  portion  of 
the  meniscus.  A  convenient 
device  for  illuminating  the  me- 
niscus is  a  visiting  card,  half  of 
which  has  been  covered  with  a 
strip  of  black  paper.  This  is 
held  against  the  back  of  the 
burette  with  the  edge  of  the 
black  portion  just  below  the 
level  of  the  meniscus.  It  is 
convenient  to  keep  the  card 
attached  to  the  burette  by  a 
little  rubber  band  (Fig.  17). 

Another  method  of  illuminat- 
ing the  meniscus  is  to  hold  a  white  card  behind  the 
burette,  as  shown 
at  M  in  Fig.  18. 

(5)  The  eye  must  be 

at  the  same  level 
as  the  surface. 
The  error  intro- 
duced by  neglect 
of  this  point  will  M 

be   understood 
from  Fig.   18,   as 
it  is  evident  that  readings  from  positions  A  and  B 
are  incorrect. 

When  washing  out  a  burette,  do  not  close  the  end  with 
the  finger,  as  this  makes  the  glass  greasy.  Either  use  a  cork, 
or  wash  by  simply  tilting  the  burette  up  and  down.  Before 
filling  a  burette  it  must  be  rinsed  out  with  a  little  of  the 
solution,  as  otherwise  the  solution  would  be  diluted  by  the 
water  left  on  the  sides  after  washing.  It  will  be  found  a 
convenience  in  filling  a  burette  to  open  out  the  upper  end 
into  a  funnel,  as  shown  in  Fig.  16. 


3-E 


,-  A 


B 


FIG.  i 8. 


CALIBRATION  OF  A  BURETTE  37 

In  a  titration  the  burette  should  be  read  to  ooi  c.c.  This 
means  that  the  reading  must  be  made  to  one-tenth  of  the 
smallest  scale-division.  It  is  often  possible  to  judge  the 
end-point  of  a  titration  to  within  ooi  c.c,  and 
in  such  cases  only  fractions  of  a  drop  should 
be  added  when  near  the  end.  This  is  done  by 
removing  the  liquid  as  it  collects  at  the  nozzle 
of  the  burette  by  touching  with  the  stirring- 
rod. 

Schellbach  Burette. — Burettes  are  some- 
times made  with  a  broad  white  band  along  the 
back  of  the  burette  and  a  narrow  (usually  blue) 
band  along  the  middle  of  the  white  one.  The 
level  is  then  very  easily  read  (Fig.  19). 

The  error  due  to  parallax  is  lessened  by  this  device.  . 


Calibration  of  a  Burette. 

If  used  properly,  burettes  are  usually  sufficiently  accurate 
for  ordinary  work,  but  some  check  is  always  desirable  as 
imperfect  instruments  are  supplied  occasionally  even  by 
good  makers. 

The  errors  in  burettes  are  of  three  types.  The  most 
common  is  an  error  in  the  total  volume,  the  error  being 
evenly  distributed  over  the  whole  range.  (It  may  be 
mentioned  that  an  apparent  error  in  the  total  volume  is 
sometimes  due  to  not  allowing  sufficient  time  for  the  burette 
to  drain.)  Sometimes  the  tube  tapers,  and  there  is  therefore 
a  progressive  error  in  the  graduations  although  the  total 
volume  may  be  correct.  A  serious  error  due  to  an  irregu- 
larity in  one  portion  of  the  burette  is  rarely  found  in  a 
modern  burette. 

Calibration. — Before  starting  to  calibrate  a  burette,  read 
the  notes  on  the  use  of  a  burette.  The  following  method 
will  in  most  cases  disclose  any  serious  error  in  the 
graduation. 

The  burette  must  be  thoroughly  clean,  and  it  is  assumed 
that  the  operator  is  familiar  with  the  correct  method  of  using 
a  burette,  since  obviously  any  method  of  calibration  is 


38 


VOLUMETRIC  ANALYSIS 


useless  if  the  instrument  is  not  manipulated  in  the  correct 
manner. 

1.  Fill  the  clean  burette  with  distilled  water  and  adjust  to 

the  zero  mark.  Run  out  the  water  slowly  into  a  tared 
weighing-bottle  until  the  last  graduation  is  reached. 
Find  the  weight  of  the  water  and  its  temperature, 
and,  using  the  data  on  p.  32,  calculate  the  volume 
of  the  water  delivered  by  the  burette.  Repeat 
the  experiment  until  concordant  results  are  obtained. 
(If  the  results  are  discordant,  either  the  burette  is 
not  clean  or  there  is  some  fault  in  manipulation.) 

The  error  in  the  total  volume  should  not  exceed 
i  in  1000. 

2.  To  test  the  uniformity  of  the  graduations,  start  again 

from  the  zero  and  run  out  into  the  weighing-bottle 
portions  of  5  c.c.  at  a  time,  weighing  each  portion 
before  addition  of  the  next.  Calculate  from  each 
weight  the  volume  corresponding  to  it  by  means  of 
the  data  on  p.  32. 

Example. — Temperature  of  water  was  14°. 

From  the  table  it  is  found  that  the  volume  of  I  gram  of 
water  at  14°,  weighed  in  air  with  brass  weights,  is  1-0018  c.c. 


Last  reading 
on  burette. 

Total  weight  of 
water  run  out. 

True 
volume. 

i  Total  error  at 
last  reading. 

5-00 

4-94 

4-95 

-0-05 

IO-00 

9-89 

9-91 

-0-09 

15-00 

14-86 

14-89 

-0-1  1 

20-00 

19-84 

19-88 

-0-12 

25-00 

24-82 

24-87 

-0-13 

30-00 

29-82 

29-87 

-0-13 

35-00 

34-83 

35-90 

-0-10 

40-00 

39-8? 

39'94 

-0-06 

45-00 

44-87 

44-95 

-0-05 

50-00 

49-87 

49-96 

-0-04 

It  is  usually  more  convenient  to  express  the  results  by  a 
curve  showing  the  error  at  any  particular  burette  reading. 
The  curve  for  this  burette  is  shown  below. 


STANDARDISATION  OF  A  PIPETTE 


39 


From  the  curve  the  correct  reading  may  be  obtained  at 
once.  For  instance,  if  the  liquid  was  run  out  from  2-00  c.c. 
to  16-45  c.c.,  the  corrected  readings  are  (2-00  —  0-02)  =  1-98 


0-15 


o-io 


0-05 


.-<)' 


--() 


-*> 


10 


20 


30 


40 


c.c.,    so    that    the    volume 


c.c.    and    (16-45—0-11)  =  16-34    uc-i 
delivered  was  16-34  —  1-98  =  14-36  c.c 

In  general  it  is  advisable  to  reject  a  burette  if  there  are 
serious  errors,  and  particularly  if  there  are  marked  irregular- 
ities in  it.  If  the  errors  are  small  and  fairly  regular,  they 
may  be  neglected  in  all  ordinary  work,  as  in  many  cases  the 
errors  are  partly  eliminated  from  the  final  result. 


USE  AND  STANDARDISATION  OP  A  PIPETTE. 

The  pipette  should  first  be  examined  to  make  sure  that 
it  is  capable  of  being  made  an  instrument  of  precision.  The 
narrow  tubes  should  not  be  above  3  mm.  internal  diameter 
for  a  10  c.c.  pipette,  or  5  mm.  for  larger  sized  pipettes.  The 
shoulder  should  slope  gradually  to  the  bulb,  as  liquid  is 
likely  to  be  retained  in  the  corner  if  it  be  badly  shaped.  A 
pipette  will  not  deliver  constant  amounts  if  it  discharges  in 
less  than  thirty  seconds.  If  it  discharges  in  any  shorter 
time,  the  aperture  should  be  lessened  by  heating  carefully  in 
a  flame  until  of  the  desired  size. 

The  volume  which  a  pipette  will  deliver  varies  with  the 
conditions  of  use,  and  it  will  only  deliver  the  same  volume 
when  the  conditions  are  kept  exactly  the  same.  The  con- 
ditions under  which  the  pipette  is  standardised  must  there- 
fore be  exactly  those  under  which  it  is  ordinarily  used. 


\ 


40  VOLUMETRIC  ANALYSIS 

1.  The  pipette  must  be   absolutely  clean.     Any  sign  of 

drops  on  the  surface  of  the  glass  indicates  that  the 
pipette  is  not  clean. 

2.  Hold  the  pipette  at  an   angle   of  about  45°   to    the 

perpendicular,  with   the  tip  touching  the 
side  of  the  receiving  vessel. 

3.  The  time  of  continuous  discharge  should  be 

about  thirty  seconds. 

4.  It  will  be  noticed  that  after  the  continuous 

run,  a  further  drop  collects  at  the  end. 
Wait  always  the  same  time  (e.g.  about  ten 
seconds),  and  then  draw  the  end  of  the 
pipette  along  the  wet  surface  of  the  vessel. 
A  small  portion  of  the  liquid  will  always 
remain  in  the  nozzle  of  the  pipette  even 
after  this  draining,  but  this  should  not  be  c.c. 
expelled. 

Standardisation. — Find  the  weight  of  an  empty       V 
weighing-bottle.     The  degree  of  accuracy  usually        \ 
desired  in  a  pipette  is  one  in  a  thousand,  so  that 
it  is  quite  sufficient  in  the  case  of  a  10  c.c.  pipette 
to  weigh  to  0-005  gram. 

Fill  the  pipette  to  the  mark  with  distilled 
water,  run  the  contents  into  the  weighing-bottle, 
and  re-weigh. 

Bottle  +  water  =   35-215  grams. 

Tare  of  weighing-bottle   =   25-170      „ 

Weight  of  water  =    10-045       » 

Find  the  temperature  of  the  water  used  and, 
from  the  table  on  p.  32,  calculate  the  volume  which 
the  observed  weight   of  water  occupies.      If  the     FlG<  20> 
error    in    a    10    c.c.    pipette    is    not    more    than    A  plPette< 
±   o-oi  c.c.,  the  pipette  is  sufficiently  accurate.     The  differ- 
ence found  will  usually  be  greater  than  this,  and  in  such 
cases  it  must  be  corrected  in  the  following  manner. 

A  rough  estimate  as  to  how  far  to  place  the  new  mark 
from  the  old  graduation  may  be  obtained  by  noting  how  far 
the  water  sinks  in  the  narrow  tube  when  one  drop  is  run  out 


STANDARD  SOLUTIONS  41 

of  the  pipette.  One  drop  of  water  weighs  somewhere  about 
0-05  gram.  Gum  a  strip  of  paper  along  the  pipette  and 
make  a  mark  where  it  is  thought  the  graduation  should  be. 

Weigh  the  amount  delivered  by  the  pipette  with  this 
new  graduation,  and  calculate  the  volume  to  which  this 
corresponds.  If  this  is  not  quite  correct,  it  will  serve  as  a 
guide  to  the  exact  position.  When  it  is  certain  (by  a 
repetition  of  the  weighing)  that  a  mark  on  the  paper  is  the 
correct  position  for  the  graduation,  a  line  should  be  etched 
on  the  glass  at  this  place.  Wash  off  the  etching  solution  and 
examine  a  small  portion  of  the  line  to  be  sure  that  it  is 
properly  etched,  before  removing  the  paper  with  the 
graduation  mark. 

A  25  c.c.  pipette  is  sufficiently  accurate  if  the  error  is  not 
more  than  ±0-025  c.c. 

GENERAL  NOTES  ON  THE  PREPARATION  OP 
STANDARD  SOLUTIONS. 

For  obvious  reasons,  it  is  desirable  that  the  standard 
solution  should  not  alter  in  concentration  in  keeping ;  volatile 
or  unstable  substances  are  therefore  to  be  avoided  if  possible. 

Solutions  more  concentrated  than  normal  are  rarely 
required  in  analytical  work.  More  dilute  solutions  may  be 
prepared  with  accuracy  from  a  standard  N  solution  by 
using  a  standardised  pipette  and  flask. 

The  solution  or  solid  from  which  the  standard  solution  is 
to  be  prepared  should  in  general  be  washed  into  the  standard 
flask  through  a  funnel.  The  flask  must  of  course  be  clean, 
but  it  is  unnecessary  to  dry  it.  Standard  flasks  should  not 
be  heated.  If  it  is  necessary  to  apply  heat  in  the  preparation 
of  the  solution,  this  operation  should  be  performed  in  a 
beaker,  and  the  solution  then  cooled  before  pouring  into  the 
flask.  If  the  solution  is  prepared  in  a  beaker  or  other  vessel, 
the  volume  of  liquid  must  be  such  that  ample  wash-water  may 
subsequently  be  used  without  exceeding  the  volume  of  the 
standard  flask. 

Before  making  up  to  the  mark  the  contents  of  the  flask 
must  be  sufficiently  mixed,  to  ensure  that  there  is  no  con- 
siderable difference  in  concentration  between  the  top  and 


42  VOLUMETRIC  ANALYSIS 

bottom  layers.  To  accomplish  this,  the  contents  of  the 
flask  are  mixed  by  rotation  when  the  level  is  a  little  below 
the  neck.  If  this  is  not  done,  an  error  is  introduced  by  the 
change  of  volume  which  occurs  when  mixing  does  take  place. 

The  solution  must  be  at  or  near  15°  before  making  the 
volume  up  to  the  graduation  mark.  Attention  to  this  is 
specially  necessary  when,  as  with  sulphuric  acid  or  sodium 
hydroxide,  there  is  a  considerable  heat  evolution  on  solution. 
In  most  cases  it  is  advisable  to  dilute  almost  to  the  full 
amount  and  cool  by  running  tap-water  over  the  flask.  The 
final  addition  of  water  to  bring  the  level  up  to  the  graduation 
mark  is  most  easily  made  from  a  wash-bottle  with  a  fine  jet. 

Before  pouring  any  of  the  liquid  out  of  the  flask,  the 
stopper  should  be  firmly  inserted  and  the  contents  thoroughly 
mixed  by  inverting  the  flask  several  times.  This  must  on 
no  account  be  neglected,  as  otherwise  serious  errors  will 
be  introduced. 

The  solution  is  best  stored  in  a  stoppered  bottle.  The 
bottle  must  be  washed  out  two  or  three  times  with  small 
quantities  of  the  solution,  the  portions  used  for  washing 
being  rejected.  The  bottle  should  be  clearly  labelled  with 
the  name  of  the  solution,  the  exact  concentration,  and  the 
date  of  preparation  or  standardisation.  If  there  is  more 
than  one  common  method  for  the  standardisation  of  the 
solution,  the  label  should  indicate  which  method  was 
adopted. 


Hydrochloric  acid 

1-017  N  (by  Calcspar) 

29/3/12 


When  a  bottle  of  standard  solution  has  been  set  aside 
for  some  time,  drops  of  liquid  will  be  found  to  have  con- 
densed on  the  upper  part  of  the  bottle.  As  these  drops 
may  differ  in  concentration  from  the  main  portion  of  the 
liquid,  it  is  always  advisable  to  shake  the  bottle  before  use. 


STANDARD  SOLUTIONS  43 

Preparation  of  a  Standard  Solution  of  a  desired 
Concentration. 

The  usual  practice  in  the  preparation  of  a  standard  solu- 
tion is  to  make  the  solution  of  approximately  the  required 
concentration,  then  to  determine  the  exact  value,  and  use 
this  value  as  a  "  factor  "  in  the  calculations.  It  is  sometimes 
necessary,  however,  to  prepare  a  solution  which  is,  for 
example,  exactly  normal,  or  decinormal,  and  the  following 
is  an  illustration  of  a  method  that  may  be  used. 

Preparation  of  an  exactly  Normal  Solution.  —  In  prepar- 
ing the  solution  it  is  better  to  make  it  somewhat  too 
concentrated  at  first  rather  than  too  dilute,  as  it  is  an  easier 
matter  to  alter  the  concentration  by  addition  of  water  than 
by  addition  of  weighed  quantities  of  the  solute. 

Example.  —  A  solution  of  sodium  carbonate  was  found  by 
titration  to  be  1-045  N.  It  is  clear  that  1000  c.c.  of  the 
solution  contains  1-045  equivalents,  and  that  it  could  be  made 
exactly  normal  by  increasing  the  volume  to  1045  c.c.  If 
45  c.c.  of  water  were  added  to  I  litre  of  the  solution,  there 
would  then  be  1-045  equivalents  in  about  1045  c-0-1 

The  volume  of  the  solution  should  be  determined  by 
means  of  a  measuring  cylinder  ;  suppose,  for  example,  the 
volume  of  the  sodium  carbonate  solution  was  found  to  be 
440  c.c.  The  amount  of  water  to  be  added  is  then 


1000 

The  19-8  c.c.  of  water  is  run  out  from  a  burette  into  the 
solution  in  the  measuring  cylinder.  (Take  care  to  add 
too  little  rather  than  too  much  water.)  The  solution  is 
poured  back  into  the  bottle,  shaken,  and  again  titrated  with 
standard  acid.  If  it  is  still  too  concentrated,  again  add  the 
calculated  amount  of  water  and  again  titrate.  After  the 
second  adjustment  the  solution  should  be  of  the  required 
concentration  ;  if  not,  continue  the  process.  After  adding 
any  water,  always  return  the  solution  to  the  bottle  and 
shake  it  before  titrating  it  with  the  acid. 

1  Addition  of  45  c.c.  of  water  will  not  give  exactly  1045  c.c.  The 
volume  obtained  when  a  solution  is  diluted  with  water  is  slightly  less 
than  the  sum  of  the  volumes  taken,  but  the  difference  is  so  small  that  it 
may  be  neglected  except  with  concentrated  solutions. 


Acidimetry  and  Alkalimetry 

THE  determination  of  the  concentration  of  acids  by  means  of 
standard  alkali  solutions  is  known  as  acidimetry,  and  the 
reverse  process  as  alkalimetry.  The  choice  of  acids  and 
alkalis  for  standard  solutions  is  partly  a  matter  of  convenience, 
but  for  certain  purposes  the  choice  is  restricted,  e.g.,  a 
standard  sodium  carbonate  solution  cannot  be  used  instead 
of  a  standard  sodium  hydroxide  solution  for  the  determina- 
tion of  acetic  acid. 

Standard  Acids. — The  only  acids  in  common  use  are 
hydrochloric  and  sulphuric  acids.  The  preparation  of  normal 
solutions  of  each,  with  various  methods  of  standardisation, 
will  be  found  below ;  more  dilute  standard  solutions  are 
best  prepared  by  dilution  of  the  normal  solution. 

For  almost  all  purposes  it  is  immaterial  which  acid  is 
used,  but  probably  hydrochloric  acid  has  the  wider  range 
of  utility.  Normal  hydrochloric  acid  is  quite  stable,  and  has 
no  tendency  to  lose  the  free  acid ;  indeed,  if  exposed  to 
the  air,  it  becomes  more  concentrated. 

Standard  Alkalis.  —  Sodium  hydroxide,  sodium  car- 
bonate, and  barium  hydroxide  are  all  used  for  standard 
solutions,  and  each  has  advantages  over  the  others  for 
special  purposes.  The  selection  of  an  indicator  is  of  special 
importance  in  connection  with  standard  alkalis ;  and  it  is 
essential  that,  in  standardising  and  in  subsequently  using 
a  standard  alkali,  one  and  the  same  indicator  should  be 
used. 

NOTES  ON  THE  USE  OP  INDICATORS. 

A  point  of  some  importance  in  connection  with  the  use 
of  indicators  in  acidimetry  and  alkalimetry  requires  special 
mention.  If  an  acid  and  alkali  are  titrated  against  one 
another,  the  observed  titre  depends  on  the  nature  of  the 
indicator.  In  the  case  of  a  strong  acid  and  a  strong  alkali 
(free  from  carbonate)  the  variation  is  negligibly  small,  unless 


NOTES  ON  INDICATORS  45 

the  solutions  are  very  dilute.  Considerable  differences  may 
be  found,  however,  with  sulphuric  acid  and,  more  especially, 
with  sodium  hydroxide,  both  of  which  ordinarily  contain 
varying  amounts  of  carbonate.  If,  therefore,  a  standard 
solution  of  either  sulphuric  acid  or  sodium  hydroxide  is 
intended  for  use  with  more  than  one  indicator,  it  should  be 
standardised  separately,  using  each  indicator,  and  the  appro- 
priate value  for  the  normality,  corresponding  to  each 
indicator,  used  in  subsequent  work. 

Indicators  for  acid  and  alkali  are  supposed  to  indicate 
the  point  of  neutrality.  They  never  really  do  so  in  practice, 
but  merely  change  colour  when  the  concentration  of  acid  or 
alkali  has  fallen  beneath  a  certain  value,  which  differs  in  the 
case  of  different  indicators.  If,  however,  attention  is  paid  to 
the  following  instructions,  sufficiently  accurate  results  are  got 
for  all  ordinary  purposes. 

If  the  solution  used  in  the  titration  contains  neither  weak 
acid  (e.g.  carbonic  or  acetic)  nor  weak  base  (e.g.  ammonia), 
it  is  immaterial  which  indicator  is  employed. 

Litmus  may  be  used  for  the  titration  of  soluble 
hydroxides,  such  as  sodium,  calcium,  and  ammonium 
hydroxide,  in  cold  solution.  It  is  inaccurate  in  presence  of 
carbonates. 

It  may  be  used  in  cold  solution  for  titration  of  nitric, 
sulphuric,  hydrochloric,  and  oxalic  acids,  but  must  not  be  used 
for  weak  acids  such  as  acetic  acid. 

Litmus  (boiling)  may  be  used  in  the  titration  of  carbon- 
ates, bicarbonates,  and  sulphides  of  the  strong  alkalis.  The 
carbon  dioxide  or  hydrogen  sulphide  is  expelled  by  boiling 
during  the  operation. 

Methyl  Orange  (cold). — Titration  with  methyl  orange 
gives  all  the  alkali  present  as  hydroxide,  carbonate,  bicarbon- 
ate, sulphide,  silicate,  borate,  or  arsenite.  As  carbon  dioxide 
is  not  entirely  without  influence  on  methyl  orange,  it  is 
advisable  in  the  titration  of  carbonates  to  expel  most  of  the 
carbon  dioxide  when  near  the  neutral  point  by  warming  and 
shaking  the  solution  ;  the  solution  must  then  be  cooled  before 
completing  the  titration. 

Methyl  orange  must  not  be  used  for  weak  acids.  It  may 
be  used  for  ammonia,  but  methyl  red  gives  a  sharper  end-point. 


46  VOLUMETRIC  ANALYSIS 

Methyl  Orange  (hot). — Titration  of  hot  solutions  gives 
inaccurate  results,  and  the  process  is  therefore  to  be 
avoided. 

Methyl  Red. — This  is  the  best  indicator  for  the  titration 
of  ammonia  or  for  solutions  containing  ammonium  salts.  It 
is  more  sensitive  and  gives  a  sharper  end-point  than  methyl 
orange  with  very  dilute  solutions  of  strong  acids.  It  is  useless 
for  weak  acids  like  acetic  acid.  Soluble  carbonates  may  be 
titrated  if  the  solution  is  heated  to  the  boiling  point.  Methyl 
red  is,  however,  much  less  sensitive  to  carbonic  acid  than 
litmus  or  phenolphthalein,  and  the  amount  of  carbonate 
present  in  an  ordinary  solution  of  sodium  hydroxide  is 
practically  without  influence  on  the  indicator. 

Phenolphthalein, — This  indicator  should  only  be  used  with 
strong  alkalis  free  from  carbonate.  Its  chief  value  lies  in  the 
fact  that  weak  acids  may  be  titrated  by  its  use,  and  that 
titrations  may  be  carried  out  in  solutions  containing  alcohol. 
Thus,  organic  acids  insoluble  in  water  may  be  titrated  in 
aqueous  alcohol  solutions  with  this  indicator. 

It  must  never  be  used  in  presence  of  ammonia  or 
ammonium  salts. 

With  carbonates  of  the  alkalis  it  indicates  "  neutrality  " 
roughly  at  the  stage  of  bicarbonate.  In  practice  it  is  always 
employed  in  the  cold. 

Amount  of  Indicator  to  be  Used. 

It  is  a  common  mistake  to  use  too  much 
indicator,  probably  because  the  solutions  are 
often  too  concentrated.  The  concentration  of 
each  indicator  solution  should  be  such  that 
maximum  sensitiveness  is  obtained  when  i  c.c. 
of  the  indicator  solution  is  added  for  each 
50  c.c.  of  the  liquid  present  at  the  end  of 
the  titration.  One  c.c.  of  the  indicator  can  be 
added  conveniently  and  with  sufficient  exact- 
ness by  means  of  a  rough  pipette  (Fig.  21) 
graduated  to  deliver  I  c.c. 

The    preparation    of  these  dilute  indicator 
solutions  is  described  in  the  Appendix. 


STANDARD  HYDROCHLORIC  ACID  47 

NORMAL  HYDROCHLORIC  ACID. 

(36-47  grams  HCl per  litre.} 

Standard  hydrochloric  acid  may  be  prepared  (i)  by 
dilution  of  a  known  volume  or  weight  of  "constant  boiling- 
point  "  acid  (see  p.  51),  or  (2)  by  preparation  of  a  solution 
of  approximately  the  desired  concentration  followed  by 
standardisation  by  one  of  the  methods  given  below. 

Commercial  concentrated  hydrochloric  acid  is  usually 
about  10  N.  Prepare  approximately  normal  acid  by 
diluting  100  c.c.  of  the  concentrated  acid  to  a  litre. 
Standardise  this  acid  by  one  or  other  of  the  follow- 
ing methods: — (i)  By  means  of  calcspar;  (2)  by  means 
of  Na2CO3,ioH2O ;  (3)  by  means  of  anhydrous  sodium 
carbonate. 

Standardisation  of  Hydrochloric  Acid  by  means  of 
Calcspar. 

It  is  important  to  have  a  method  whereby  the  accuracy 
of  a  standard  acid  solution  may  be  checked,  or  the  concentra- 
tion of  an  unknown  solution  accurately  determined.  It  is 
necessary  for  this  purpose  to  find  a  substance  which  will 
conform  to  certain  requirements. 

It  must  be  possible  to  find  the  concentration  of  the  acid 
readily  and  accurately  by  use  of  this  standard  substance. 
The  substance  must  be  readily  obtainable  in  a  state  of  purity. 
It  should  be  non-volatile  and  non-deliquescent,  to  ensure 
accuracy  in  weighing.  Calcspar  fulfils  these  conditions. 

First  method. — Select  a  good  piece  of  calcspar  weighing 
from  3  to  5  grams.  Place  it  in  a  beaker  and  cover  with 
dilute  hydrochloric  acid  for  two  or  three  minutes  to  remove 
the  fine  powder  on  its  surface.  Wash  well  with  water, 
remove  most  of  the  water  with  filter  paper,  and  then  dry  in 
the  steam-oven  for  half  an  hour. 

Clean  and  dry  a  small  (100  c.c.)  beaker  or  conical  flask. 
Place  the  prepared  piece  of  calcspar  in  this,  and  find  the 
weight  of  the  vessel  plus  calcspar.  If  a  beaker  is  used, 
cover  it  with  a  watch-glass  ;  if  a  flask,  it  should  be  protected 
as  shown  in  Fig.  9  (p.  21).  Run  in  from  a  pipette  25  c.c.  of 


48  VOLUMETRIC  ANALYSIS 

the  acid  to  be  standardised,  keeping  the  cover  in  position 
as  far  as  possible  to  prevent  loss  due  to  the  effervescence. 
Set  aside  for  several  hours  or  overnight. 

Wash  down  the  sides  and  cover  of  the  vessel,  add  I  c.c. 
of  methyl  orange,  and  warm  on  the  steam-bath  until  the 
solution  becomes  neutral  (yellow  colour). 

Pour  off  the  calcium  chloride  solution,  and  wash 
thoroughly  by  decantation,  taking  care  that  there  is  no  loss 
of  any  small  pieces  of  the  spar  which  have  become  detached 
from  the  main  portion.  Drain  off  as  much  of  the  water  as 
possible  and  dry  in  the  steam-oven  again,  without  removing 
the  calcspar  from  the  beaker.  Cool  and  weigh. 

From  the  loss  in  weight,  calculate  the  concentration  of 
the  hydrochloric  acid. 

Example : — 

Original  weight  of  beaker  and  spar     .     16-5124  grams 
Final  „  „  „        .     15-2514      „ 

Weight  of  CaCO3  dissolved  by  25  cc.        1-2610     „ 

Therefore    1000  c.c.  of  the  acid   will   dissolve    50-44  grams 

CaCO3. 
But  IOOO  c.c.  normal  acid  will  dissolve  50-03  grams  CaCO3. 

Therefore  the  given  acid  is  - — —  =  1-008  normal. 

50-03 

Note. — The  solubility  of  glass  in  water  and  in  acid  and 
alkaline  solutions  occasions  an  appreciable  error  in  this  and 
similar  experiments  in  acidimetry.  The  error  is  considerable 
in  the  case  of  glass  which  has  not  been  in  use  for  some  time. 
To  minimise  the  error,  the  flask  or  beaker  in  the  above 
experiment  should  be  "  steamed,"  i.e.,  steam  should  be  blown 
through  it  for  ten  minutes  before  finally  rinsing  it  out  for 
use. 

Alternative  method. — Clean  and  dry  a  small  beaker  of 
about  100  c.c.  capacity,  together  with  a  watch-glass  to  cover 
it.  Weigh  the  beaker  and  cover,  and  then  weigh  in  it  from  I 
to  1-2  grams  of  powdered  calcspar,  e.g., 

Beaker,  watch-glass  and  calcspar    =    17-009  grams 
Tare  of  beaker  and  watch-glass       =    15-998      „ 

Weight  of  calcspar  =      i-oii      „ 


STANDARD  HYDROCHLORIC  ACID  49 

Cover  the  calcspar  with  water  and  run  in  from  a  pipette 
25  c.c.  of  the  hydrochloric  acid,  keeping  the  beaker  covered 
as  much  as  possible  to  prevent  loss  during  the  effervescence. 
After  a  few  minutes  the  solution  may  be  warmed  gently  to 
complete  the  reaction,  but  should  not  be  boiled. 

Wash  down  the  sides  of  the  beaker  and  the  cover-glass. 
Titrate  the  unused  acid  with  an  alkali  solution,  using  i  c.c. 
of  methyl  orange  as  indicator.  It  is  not  necessary  to  know 
the  concentration  of  this  alkali,  but  its  titration  value  against 
the  acid  must  be  known.  The  amount  of  unused  acid  is  then 
calculated.  Subtraction  of  this  from  the  25  c.c.  taken  gives 
the  volume  neutralised  by  the  calcspar,  and  from  this  the 
concentration  of  the  acid  is  calculated. 

Example. — i-oii  grams  of  calcspar  were  dissolved  in  25 
c.c.  of  hydrochloric  acid  solution  and  6-O  c.c.  of  alkali  were 
used  in  the  back  titration,  using  methyl  orange  as  indicator. 
Twenty-five  c.c.  of  this  alkali  neutralised,  using  the  same 
indicator,  24-0  c.c.  of  the  acid.  The  6-0  c.c.  of  alkali  correspond, 

/ >f)  A  \ 

therefore,  to  f  —  x  6J  c.c.  =  576  c.c.  of  acid. 

Therefore  i-oii  grams  of  CaCO3  neutralised  (25  —  576) 
=  19-24  c.c.  of  acid.  The  hydrochloric  acid  is  1-050  N. 

Standardisation  of  Hydrochloric  Acid  by  means  of 
Sodium  Carbonate  Crystals,  Na2CO3,  10H2O. 

The  sodium  carbonate  must  be  free  from  sulphate  and 
chloride,  and  crystals  that  show  any  traces  of  efflorescence 
must  be  rejected. 

Place  about  10  grams  of  selected  crystals  in  a  weighing- 
bottle  and  weigh  to  the  nearest  milligram.  Transfer  3  to  3-5 
grams  to  a  200  c.c.  beaker  or  conical  flask  and  reweigh  the 
bottle  to  find  the  weight  of  sodium  carbonate  taken.  Dissolve 
the  crystals  in  about  50  c.c.  of  water,  add  2  c.c.  of  methyl 
orange,  and  titrate  with  the  hydrochloric  acid.  Shake  and 
warm  the  solution  when  near  the  neutral  point  to  expel 
most  of  the  carbon  dioxide,  but  cool  again  to  room  tempera- 
ture before  finishing  the  titration.  Calculate  the  concentration 
of  the  acid,  and  repeat  the  process  until  results  agreeing  to 
within  2  in  1000  are  obtained. 

Alternative  method. — To  a  weighed  quantity  of  the 

D 


50  VOLUMETRIC  ANALYSIS 

crystals  add  a  measured  volume  of  hydrochloric  acid,  taking 
the  acid  in  slight  excess  (3  to  3-2  grams  of  Na2CO3,ioH2O  and 
25  c.c.  of  approximately  N  acid  are  convenient  quantities). 
Boil  the  solution  for  a  minute  to  expel  the  carbon  dioxide, 
and  titrate  the  unused  acid  with  an  alkali  solution,  using 
methyl  orange  or  methyl  red  as  indicator.  The  method  of 
calculation  is  explained  for  a  similar  case  on  p.  49.  The 
end-point  obtained  by  this  method  is  sharper  than  in  the 
direct  titration  method,  the  difference  being  more  noticeable 
with  decinormal  than  with  normal  solutions. 

Standardisation  of  Hydrochloric  Acid  by  means  of 
Anhydrous  Sodium  Carbonate. 

The  sodium  carbonate  required  for  this  purpose  must 
answer  the  following  tests  : — 

Dissolve  2  or  3  grams  in  distilled  water ;  the  solution 
must  be  perfectly  clear.  Add  nitric  acid  (free  from  chloride) 
until  effervescence  ceases  and  the  solution  is  slightly  acid. 
To  one  half  of  the  solution  add  silver  nitrate :  no  turbidity 
(or  only  a  very  slight  opalescence)  should  appear.  Dilute 
the  remainder  considerably,  add  barium  nitrate,  and  boil : 
no  trace  of  barium  sulphate  should  be  precipitated  (if  in- 
sufficiently diluted,  barium  nitrate  may  be  precipitated). 

If  the  sodium  carbonate  is  free  from  chloride  and  sulphate, 
and  from  insoluble  impurities,  it  must  next  be  dried.  Heat 
3  or  4  grams  in  a  small  porcelain  basin  (a  platinum  crucible 
is  preferable)  over  a  moderate  Bunsen  flame,  with  frequent 
stirring,  for  twenty  minutes ;  regulate  the  flame  so  that  the 
bottom  of  the  crucible  is  barely  red-hot ;  the  substance  must 
not  fuse  or  form  into  hard  lumps.  Allow  to  cool  partially, 
and  transfer  whilst  still  warm  to  a  clean  dry  weighing-bottle. 

In  order  to  avoid  over-heating  the  carbonate,  the  basin 
or  crucible  may  be  heated  in  a  sand-bath  to  300°  for  thirty 
minutes,  with  frequent  stirring. 

When  the  weighing-bottle  is  perfectly  cold,  weigh  it 
accurately,  shake  out  from  I  to  1-2  grams  of  the  carbonate  into 
a  suitable  beaker  or  conical  flask,  and  weigh  again.  Shake 
out  a  second  quantity  into  another  beaker,  and  again  weigh. 

Dissolve  each  portion  of  carbonate  in  about  30  c.c.  of 
water,  add  i  c.c.  of  methyl  orange,  and  titrate  with  the. 


STANDARD  HYDROCHLORIC  ACID  51 

hydrochloric  acid.     Calculate  the  concentration  of  the  acid 
as  follows  : — 

1-251  grams  Na.2CO3  required  23-52  c.c.  HC1.     Therefore 
1000  c.c.  HC1  are   equivalent  to  — — — —  —  =  53-17  grams 
Na2CO3.     But  1000  c.c.  normal  HC1  are  equivalent  to  53-0 
grams  Na2CO3.     Therefore  the  acid  is  ~ — -=1-003  N. 
The  two  results  should  agree  to  about  2  in  1000. 

Sodium  bicarbonate,  free  from  chloride  and  sulphate  and 
insoluble  impurities,  may  be  used  for  the  preparation  of  the 
normal  carbonate.  For  this  purpose,  water  and  carbon 
dioxide  are  driven  off  by  heating  to  a  moderate  temperature, 
as  in  the  case  of  sodium  carbonate,  in  a  platinum  crucible 
(or  a  porcelain  dish)  for  thirty  minutes,  with  frequent  stirring. 
The  normal  carbonate  is  used  as  described  above. 


Preparation  of  Standard  Hydrochloric  Acid  Solution 
from  the  Constant  Boiling-point  Acid. 

Hydrochloric  acid,  when  distilled,  approaches  a  concentra- 
tion at  which  it  distils  unchanged  whether  one  starts  with  a 
dilute  or  concentrated  acid,  and  when  this  point  is  reached 
the  distillate  and  residue  have  a  definite  composition.  The 
composition  varies  with  the  pressure,  but  the  variation  caused 
by  any  possible  barometric  variation  is  so  small  that  it  may 
be  neglected  even  for  exact  work. 

Make  up  600  c.c.  of  hydrochloric  acid  solution  of  as  near 
MO  sp.  gr.  as  possible  with  the  use  of  a  specific  gravity  bulb. 
Boil  this  solution  in  a  narrow-mouthed  flask  until  about 
two-thirds  of  the  solution  have  been  driven  off.  The 
evaporation  must  not  be  done  in  an  open  vessel,  as  errors 
are  introduced  if  air  has  access  to  the  surface  of  the  solution. 
The  residue  is  then  cooled.  This  constant  boiling-point 
acid  contains  20-24  per  cent,  by  weight  of  hydrochloric 
acid ;  180-2  grams  made  up  to  I  litre  therefore  give  a  normal 
solution. 

The  constant  boiling  acid  is  neither  hygroscopic  nor 
noticeably  volatile,  and  may  be  weighed  in  a  small  tared 


52  VOLUMETRIC  ANALYSIS 

flask.     The  last  amount  should  be  run  in  from  a  tube  drawn 
out  to  a  fine  capillary. 

If  there  is  any  doubt  as  to  the  purity  of  the  original  acid, 
it  is  safer  to  condense  the  vapour  after  two-thirds  have  been 
boiled  off,  and  use  this  distillate  instead  of  the  residue. 


NORMAL  SULPHURIC  ACID. 

(49-04  grams  H^SO^per  litre.) 

Dilute  30  c.c.  of  ordinary  concentrated  sulphuric  acid  by 
running  it  slowly  into  150  to  200  c.c.  of  cold  water  contained  in 
a  flask.  (Caution. — The  acid  must  be  run  into  the  water, 
not  vice  versa.)  After  cooling  the  solution  under  the  tap, 
pour  it  into  a  graduated  flask  and  dilute  to  a  litre.  The 
solution  prepared  in  this  way  will  be  slightly  above 
normal ;  the  exact  concentration  must  be  found  by 
standardisation. 

Standardisation. — Sulphuric  acid  may  be  standardised 
in  exactly  the  same  manner  as  hydrochloric  acid  with 
weighed  quantities  of  sodium  carbonate  (see  pp.  49  and  50). 

Calcspar  cannot  be  used  as  a  standard  for  sulphuric  acid, 
on  account  of  the  insolubility  of  calcium  sulphate. 

Sulphuric  acid  is  often  standardised  gravimetrically  by 
precipitation  as  barium  sulphate.  Ten  c.c.  of  a  normal 
solution  or  50  c.c.  of  a  decinormal  solution  is  sufficient  for 
each  gravimetric  determination.  The  details  of  the  gravi- 
metric method  are  given  on  p.  131. 

NORMAL  SODIUM  HYDROXIDE. 

(40-01  grams  NaOH 'per  litre.) 

Sodium  hydroxide  is  very  deliquescent,  and  the  commercial 
"sticks"  contain  varying  amounts  of  carbonate.  For  most 
purposes  the  solution  made  from  ordinary  "white  sticks" 
may  be  used,  but  when  a  "carbonate -free"  hydroxide  is 
required  the  solution  must  be  prepared  by  one  of  the  special 
methods  given  below. 

Dissolve  40  grams  of  sodium  hydroxide  (sticks)  in  water 
and  dilute  to  about  I  litre.  (Use  an  ordinary  flask.)  During 


STANDARD  SODIUM  HYDROXIDE  53 

the  process  of  solution  the  liquid  should  be  kept  in  motion, 
otherwise  the  heat  evolved  may  cause  the  vessel  to  crack. 
This  solution  must  be  standardised,  and  the  method  adopted 
must  depend  on  the  future  use  of  the  solution.  If  it  is  to 
be  used  solely  in  conjunction  with  standard  acid,  e.g.,  for  the 
back-titration  in  ammonia  estimations,  it  must  be  standard- 
ised by  titration  against  standard  acid  with  methyl  orange 
or  methyl  red  as  indicator. 

If  the  sodium  hydroxide  is  to  be  used  for  the  determina- 
tion of  weak  acids,  it  must  be  standardised  against  potassium 
tetroxalate  or  succinic  acid  with  phenolphthalein  as 
indicator. 

Details  of  Standardisation. — Weigh  accurately  on  a 
watch-glass  about  2  grams  of  potassium  tetroxalate.  Wash 
it  into  a  clean  beaker,  add  about  30  c.c.  of  water  and  I  c.c.  of 
phenolphthalein  solution.  The  tetroxalate  is  not  very 
soluble  in  water ;  the  alkali  should  therefore  be  run  in  from 
time  to  time  to  neutralise  the  portion  which  has  dissolved. 
When  the  dissolved  portion  is  neutralised,  more  of  the  solid 
will  dissolve. 

Potassium  tetroxalate  has  the  formula  KHC2O4,H2C2O4, 
2H2O.  It  has  three  replaceable  H  atoms  in  the  molecule,  so 
that  the  equivalent  is  84-7. 

Example  of  Calculation. — It  is  found  that  2-178  grams  of 
potassium  tetroxalate  neutralise  28-20  c.c.  of  a  sodium 
hydroxide  solution. 

The  solution  is  ( ^^—  X  ^^ )  N  =  o-9i2  N,  and  contains 

V  o4'7        2o-2/ 

(40-01  x  0-912)  grams  NaOH  per  litre. 

Repeat  the  experiment  with  another  weighed  quantity  of 
potassium  tetroxalate,  and  calculate  the  concentration  of  the 
sodium  hydroxide  solution  from  the  result.  The  experiment 
should  be  repeated  until  results  are  obtained  which  agree  to 
2  in  1000. 

The  equivalent  of  succinic  acid  is  59-03,  and  about  1-5 
grams  should  therefore  be  taken  for  titration  against  N 
alkali. 

Decinormal  Sodium  Hydroxide. 

Decinormal  or  any  other  concentration  of  sodium 
hydroxide  solution  may  be  made  up  by  dilution  of  the 


54  VOLUMETRIC  ANALYSIS 

normal  solution  with  recently  boiled  water,  or  by  dissolving 
about  the  right  weight  of  sodium  hydroxide  in  a  known 
volume.  In  the  latter  case  the  solution  must  be  standardised 
in  a  manner  similar  to  the  above.  The  amount  of  tetroxa- 
late  taken  should  be  such  as  to  neutralise  about  25  to  30  c.c. 
of  the  solution. 


Preparation  of  Carbonate-free  Sodium  Hydroxide. 

Sodium  hydroxide  is  often  used  for  the  determination  of 
weak  acids,  with  phenolphthalein  as'  indicator.  If  any 
carbonate  is  present  the  end-point  is  very  unsatisfactory. 
Commercial  sodium  hydroxide  almost  invariably  contains 
some  carbonate  as  impurity,  and  it  is  therefore  preferable  to 
prepare  it  as  required  from  metallic  sodium.  The  following 
method  may  be  used,  except  in  the  rare  cases  where  the 
presence  of  alcohol  is  not  permissible. 

Cut  away  the  layer  of  oxide  from  the  surface  of  a  piece 
of  sodium,  using  alcohol  to  lubricate  the  knife.  Weigh  out 
approximately  23  grams  of  the  metal  and  drop  it,  in  small 
pieces  at  a  time,  into  about  25  c.c.  of  ethyl  alcohol,  contained 
in  a  porcelain  basin.  When  the  reaction  becomes  sluggish, 
it  can  be  hastened  by  cautious  addition  of  a  few  drops  of 
water.  During  the  reaction  protect  the  solution  from 
atmospheric  carbon  dioxide  by  means  of  a  clock-glass  over 
the  basin ;  and  immediately  solution  is  complete,  dilute  with 
boiled  water  to  a  litre. 

This  solution  should  be  practically  free  from  carbonate, 
but  it  will  only  remain  so  if  carefully  protected  from 
atmospheric  contamination ;  the  apparatus  described  on 
p.  62  is  recommended  for  keeping  such  solutions. 

Another  method. — Prepare  a  saturated  solution  of  sodium 
hydroxide  by  pouring  water  into  a  full  I  Ib.  bottle  of  "  white 
sticks."  It  is  as  well  to  place  the  bottle  containing  the 
solution  in  a  large  porcelain  basin,  as  it  may  break.  Set  the 
bottle  aside  for  a  few  days  until  the  precipitate  has  settled. 
The  supernatant  liquid  is  a  solution  of  pure  sodium  hydroxide, 
free  from  carbonate,  and  is  about  15  or  16  N.  To  prepare 
an  approximately  normal  solution,  65  c.c.  of  the  clear  solution 
is  diluted  to  I  litre  with  CO2-free  water. 


ACIDIMETRY  AND  ALKALIMETRY  55 

ANALYSES  INVOLVING  THE  USE  OP  STANDARD 
ACID  AND  ALKALI. 

Acetic  Acid  in  Vinegar. 

The  acidity  of  a  pure  sample  of  vinegar  is  due  almost 
entirely  to  acetic  acid,  and,  even  in  adulterated  vinegar,  other 
acids 1  are  rarely  found ;  the  total  acidity  of  the  vinegar  may 
therefore  be  attributed  to  acetic  acid.  The  amount  of  acid 
varies  very  largely,  but  is  usually  in  the  neighbourhood  of 
5  per  cent. 

The  vinegar,  whether  coloured  or  not,  is  titrated  against 
normal  sodium  hydroxide  with  phenolphthalein  as  indicator. 

Dilute  the  measured  quantity  of  vinegar  (25  c.c.)  with 
about  twice  as  much  water,  to  diminish  the  loss  of  acetic 
acid  by  volatilisation.  Add  2  c.c.  of  phenolphthalein,  and 
titrate  the  solution  at  once  with  normal  sodium  hydroxide. 
The  first  tinge  of  pink  can  be  seen  even  with  dark 
vinegars,  but  if  there  is  any  doubt  about  the  end-point 
the  colour  of  the  solution  should  be  compared  with  that  of 
the  same  amount  of  vinegar  diluted  to  the  same  extent  with 
water :  the  colour  change  is  then  very  readily  detected. 

Calculation. — Calculate  the  concentration  of  acetic  acid  in 
grams  per  litre.  It  is  customary,  however,  to  express  the 
result  in  percentage  by  weight  of  acetic  acid.  The  density 
at  15°  of  5  per  cent,  vinegar  is  about  1-019,  and  it  may  there- 
fore be  assumed,  without  serious  error,  that  I  litre  of  vinegar 
weighs  1020  grams.  On  this  basis  calculate  the  percentage 
by  weight  of  acetic  acid. 

Borax. 

Boric  acid  has  no  influence  on  methyl  orange,  and  borax 
may,  therefore,  be  titrated  against  standard  hydrochloric 
acid  with  this  indicator.  Sulphuric  acid  should  not  be  used, 
as  it  does  not  give  a  sharp  end-point  in  this  titration. 

Exercise. — Titrate  a  weighed  quantity  (about  4  grams)  of 

1  A  trace  of  sulphuric  acid  (very  rarely  above  o-i  per  cent.)  is  some- 
times present  even  in  vinegar  which  is  otherwise  untreated  in  any  way. 
It  was  at  one  time  erroneously  believed  that  this  addition  improved  the 
keeping  qualities  of  the  vinegar. 


56  VOLUMETRIC  ANALYSIS 

borax  against  N  hydrochloric  acid  with  methyl  orange  as 
indicator.    Calculate  the  percentage  of  Na2B4O7  in  the  sample. 

Solubility  of  Lime  in  Water. 

Prepare  a  N/2O  hydrochloric  acid  solution  by  diluting 
25  c.c.  of  N  acid  to  500  c.c.  with  water.  (This  solution  will 
be  one-twentieth  the  concentration  of  the  original  standard 
acid.) 

Shake  up  some  lime  with  water  in  a  stoppered  bottle 
and  set  aside  until  the  solid  has  settled.  Pipette  out  25  c.c. 
of  the  clear  supernatant  solution,  and  titrate  with  N/2O  acid, 
using  phenolphthalein  as  indicator.  From  the  result  calculate 
the  solubility  of  calcium  hydroxide  in  grams  per  litre. 

Mercury. 

When  solutions  of  mercuric  chloride  and  hydrocyanic 
acid  are  mixed,  the  following  reaction  occurs  : — 

HgCl2  +  2HCN  =  Hg(CN)2  +  2HC1. 

The  reaction  is  complete,  and  offers  a  means  of  deter- 
mining mercury,  since  mercuric  chloride,  hydrocyanic  acid, 
and  mercuric  cyanide  are  all  neutral  to  methyl  orange,  and 
the  amount  of  hydrochloric  acid  set  free  is  equivalent  to  the 
amount  of  mercury  present. 

A  solution  of  hydrocyanic  acid  neutral  to  methyl  orange 
is  also  required.  Caution. — Do  not  inhale  any  hydrocyanic 
acid,  as  it  is  extremely  poisonous. 

Dissolve  i  gram  of  potassium  cyanide  in  100  c.c.  of 
water,  add  some  barium  nitrate  solution,  and  filter  off  the 
precipitated  barium  carbonate.  This  removes  the  carbonate 
which  would  interfere  with  the  neutralisation.  To  the  filtrate 
add  2  c.c.  of  methyl  orange  as  indicator,  and  then  hydro- 
chloric acid  (about  decinormal)  until  neutral.  This  solution 
is  the  "  neutral "  hydrocyanic  acid  solution. 

The  mercury  must  be  present  as  mercuric  chloride.  If 
the  solution  contains  mercuric  nitrate,  add  a  slight  excess  of 
sodium  chloride.  Make  this  solution  neutral  to  methyl 
orange  by  addition  of  the  indicator,  followed  by  dilute 
hydrochloric  acid  or  sodium  hydroxide  as  may  be  required. 
To  this  solution  add  excess  of  the  "  neutral "  hydrocyanic 


ACIDIMETRY  AND  ALKALIMETRY  57 

acid.  Set  the  mixture  aside  for  at  least  one  hour  ;  the 
reason  for  this  is  that  the  reaction,  though  quantitative,  is 
not  instantaneous. 

Titrate  the  liberated  hydrochloric  acid  with  decinormal 
sodium  hydroxide. 

A  small  excess  of  hydrocyanic  acid  is  sufficient,  and 
on  account  of  its  poisonous  properties  an  unnecessary 
excess  is  to  be  avoided.  If  there  is  any  doubt  as  to 
whether  sufficient  has  been  added,  add  a  few  drops  after 
the  titration.  This  will  restore  the  pink  colour  if  insufficient 
has  been  present. 

Exercise. — Dissolve  a  weighed  quantity  (about  0-3  grams) 
of  mercuric  oxide  in  a  few  c.c.  of  concentrated  nitric  acid, 
dilute  to  250  c.c.  with  water,  and  determine  the  mercury  in 
portions  of  25  c.c.  of  this  solution. 

Oxide  and  Carbonate  in  Quicklime. 
Weigh  to  the  nearest  milligram  about  8  grams  of  quick- 
lime, and  grind  it  to  a  smooth  paste  with  water  in  a  glazed 
porcelain  mortar.  It  will  be  found  most  convenient  to  do 
this  in  small  portions  at  a  time.  Wash  the  paste  through  a 
funnel  into  a  250  c.c.  graduated  flask,  and  dilute  with  freshly 
prepared  distilled  water  to  the  mark.  The  carbonate  will 
remain  undissolved,  but,  if  it  has  been  finely  ground,  a  repre- 
sentative sample  of  the  lime  is  obtained  if  the  flask  is 
shaken  immediately  before  withdrawal  of  a  sample. 

(1)  Determine  the  total  alkali  as  follows: — Add  50  c.c. 
of  N   hydrochloric  acid  to   50  c.c.  of  the  solution,  and  stir 
until  the  reaction  is  complete.     Titrate  the  excess  of  acid 
with  standard  alkali,  using  methyl  orange  as  indicator. 

(2)  Determine   the   oxide   by   titration   of  25    c.c.  with 
standard    acid,    using    phenolphthalein    as    indicator.      As 
none  of  the  acid  must  be  used  by  the  suspended  calcium 
carbonate,  the  following  precautions  are  necessary.     Dilute 
the  solution  before  titration  with  about  100  c.c.  of  CO2-free 
(freshly   distilled)   water,  and    run  the   acid   in  very  slowly 
and   with   constant  stirring.     Titrate  until   the   colour  just 
disappears. 

From  the  two  titrations  calculate  the  percentage  of 
calcium  oxide  and  calcium  carbonate  in  the  quicklime. 


58  VOLUMETRIC  ANALYSIS 

Acidic  Radical  in  Salts  of  Heavy  Metals. 

The  metallic  radical  is  removed  from  a  solution  by 
precipitation  with  hydrogen  sulphide  ;  e.g., 

CuSO4  +  H2S  =  CuS  +  H3SO4. 

After  filtration  from  the  sulphide,  the  mineral  acid  liberated 
is  titrated  with  standard  alkali.  This  method  is  applicable 
to  the  determination  of  the  acidic  radical  in  many  salts  of  the 
heavy  metals.  It  is  assumed  that  no  free  acid  is  present  in 
the  original  solution  except  that  derived  from  hydrolysis  of 
the  salt. 

Exercise. — Dissolve  about  3  grams  (exactly  weighed)  of 
copper  sulphate  in  hot  water,  add  about  5  grams  of  pure 
sodium  chloride,  and  precipitate  the  copper  with  hydrogen 
sulphide.  Filter,  wash  with  hot  water,  and  boil  the  mixed 
filtrate  and  washings  until  most  of  the  hydrogen  sulphide  is 
expelled.  Cool  and  titrate  with  N  sodium  hydroxide,  using 
methyl  orange  as  indicator. 

Calculate  the  percentage  of  SO4  in  the  salt. 

Ammonia  (Indirect  Method}. 

The  following  method  is  given  more  as  an  exercise  with 
acid  and  alkali  than  as  a  method  for  the  determination  of 
ammonia,  since  its  application  is  limited  to  cases  where  the 
substance  is  neutral. 

If  an  ammonium  salt  such  as  ammonium  chloride  is 
boiled  with  a  known  volume  of  standard  alkali,  the  ammonia 
will  be  expelled  and  a  corresponding  amount  of  the  alkali 
neutralised.  This  amount  can  be  determined  by  titration  of 
the  amount  left  over,  and  subtraction  from  the  amount 
taken : 

NH4C1  +  NaOH  =  NaCl  +  NH3  +  H2O. 

Weigh  accurately  about  i  gram  of  the  substance  (e.g. 
ammonium  sulphate),  wash  it  through  a  funnel  into  a  flask, 
and  add  25  c.c.  of  standard  (N)  alkali. 

Boil  the  solution  until  no  ammonia  is  detected  in  the 
steam  when  tested  with  litmus  or  turmeric  paper.  If  the 
solution  becomes  too  concentrated  more  water  should  be 
added.  When  all  the  ammonia  has  been  expelled,  cool  the 


ACIDIMETRY  AND  ALKALIMETRY 


59 


solution,   and    titrate    the    amount    of  unused    alkali   with 
standard  acid,  using  methyl  orange  as  indicator. 

Each  gram-molecule  of  NaOH  neutralised  corresponds 
to  a  gram-molecule  of  NH3  in  the  substance. 

Calculate  from  the  results  the  percentage  of  NH3  in  the 
salt. 

Ammonia  {Direct  Method). 

OUTLINE  OF  METHOD. — The  substance  is  boiled  with  excess  of  sodium 
hydroxide  solution,  and  the  ammonia  evolved  is  absorbed  by  a 
known  volume  of  a  standard  acid  solution.  The  amount  of  acid 
neutralised  by  the  ammonia  is  then  determined  by  titration  of  the 
unused  acid  with  standard  alkali. 

Procedure. — Arrange  the  apparatus  as  shown  in  Fig. 
22.  A  is  a  copper  flask  of  at  least  500  c.c.  capacity. 
(A  glass  flask  may  be  used,  but  glass  vessels  often  break 
when  used  with  a  boiling 
alkaline  solution.)  Fit  the 
flask  with  a  two  -  holed 
rubber  cork  to  carry  the 
tap-funnel  C  and  the  tube 
leading  to  the  condenser. 
It  is  advisable  to  have  a 
trap  at  D  to  prevent  any 
drops  of  sodium  hydroxide 
being  driven  over  during 
the  boiling.  To  the  lower 
end  of  the  condenser  attach 
a  boiling-tube  with  a  hole 
in  the  bottom.  The  boiling- 
tube  should  be  so  wide  that 
it  will  just  pass  easily 
through  the  neck  of  the 
flask  B,  and  should  dip  into 
the  liquid  in  the  flask. 

Examine  the  apparatus 
carefully  to  be  sure  that 
there  is  no  leakage  at  any  of  the  corks. 

Introduce  into  the  tap-funnel  a  measured  quantity  of  the 
substance  to  be  analysed,  and  wash  it  into  the  flask  with 
water.  In  the  absorption  flask  B  place  a  measured  volume 


FIG.  22. 


60  VOLUMETRIC  ANALYSIS 

(25  or  50  c.c.)  of  standard  acid,  and  add  to  it  methyl  orange, 
or,  better,  methyl  red. 

When  these  preparations  are  completed,  run  into  the 
large  flask  through  the  tap-funnel  an  excess  of  sodium, 
hydroxide  solution,  and  close  the  tap  as  soon  as  all  the  alkali 
has  entered.  In  most  cases  about  50  c.c.  of  bench  sodium 
hydroxide  will  be  sufficient.  Add  sufficient  distilled  water 
to  ensure  that  the  contents  of  A  will  not  be  evaporated  to 
dryness  during  the  experiment. 

It  is  of  course  essential  that  all  the  ammonia  evolved 
should  be  caught  by  the  standard  acid  in  the  absorption 
flask.  The  main  danger  of  loss  is  while  the  air  in  the  large 
flask  is  being  expelled.  Apply  heat,  therefore,  cautiously,  so 
that  there  is  no  sudden  rush  of  gas  through  the  standard 
acid.  Boil  for  thirty  minutes,  by  which  time  all  the  ammonia 
should  be  over,  but  it  is  as  well  to  make  sure.  Disconnect 
at  E,  and  test  the  distillate  with  litmus  paper.  If  it  is  still 
alkaline  the  boiling  must  be  continued. 

When  all  the  ammonia  has  been  driven  over,  disconnect 
the  apparatus  and  titrate  the  solution  in  B  to  find  how  much 
of  the  acid  remains  unneutralised. 

Exercise. — In  general  the  amount  of  substance  taken  for 
analysis  must  be  regulated  by  the  results  of  qualitative 
analysis.  For  practice,  determine  the  percentage  of  NH3  in 
ammonium  sulphate  or  chloride.  Take  about  I  gram 
(accurately  weighed)  and  absorb  the  ammonia  with  25  c.c. 
of  N  acid. 

Nitrate. 

The  nitrate  is  reduced  by  means  of  iron  and  sulphuric 
acid,  all  the  nitrogen  being  obtained  as  ammonium  sulphate. 
The  ammonia  is  then  determined  in  the  usual  manner. 

Place  a  weighed  portion  (about  I  gram)  of  the  substance 
in  a  500  c.c.  conical  flask,  and  add  10  grams  of  reduced  iron 
(B.  P.  ferrum  redactum)  and  50  c.c.  of  water.  Fit  the  flask 
with  a  rubber  cork  and  reflux  condenser.  (A  reflux 
condenser  is  a  condenser  placed  upright  above  the  flask  so 
that  the  condensed  liquid  runs  back  into  the  flask.)  Add 
20  c.c.  of  a  mixture  of  two  parts  of  water  and  one  part  of 
concentrated  sulphuric  acid,  and  boil  gently  for  five  minutes. 


STANDARD  BARIUM   HYDROXIDE  61 

Remove  the  flame,  and  rinse  back  into  the  flask  any  liquid 
that  has  collected  on  the  inner  surface  of  the  condenser. 
Boil  again  for  five  minutes,  cool,  and  determine  the  ammonia 
by  the  direct  method. 

Exercise. — Determine  the  percentage  of  pure  potassium 
nitrate  in  a  commercial  specimen  of  nitre.  Use  about  I 
gram  of  the  nitre,  and  collect  the  ammonia  with  25  c.c.  of 
seminormal  acid. 

Persulphates. 

Methyl  alcohol  is  oxidised  by  alkaline  persulphates  to 
formaldehyde  according  to  the  equation  : 

K2S2O8  +  CH3OH  =  2KHSO4  +  H.CO.H. 

The  persulphate  may  be  estimated  by  titration  of  the  acid 
sulphate  produced. 

Weigh  accurately  about  0-3  gram  of  the  substance  and 
dissolve  in  about  100  c.c.  of  water.  Add  I  c.c.  of  methyl 
orange,  and,  if  the  solution  is  acid,  neutralise  with  sodium 
hydroxide.  To  the  neutralised  solution  add  2  c.c.  of  methyl 
alcohol,  heat  to  about  70°  for  five  minutes,  and  then  boil 
gently  for  a  further  ten  minutes.  Cool,  and  titrate  with 
decinormal  alkali. 

STANDARD  BARIUM  HYDROXIDE  (BARYTA) 
SOLUTION. 

(JV/20  solution  contains  7.890  grams  Ba(OH)^  Sff2O per  litre.) 

The  titration  of  acids,  using  phenolphthalein  or  methyl  red 
as  indicator,  is  accurate  only  if  made  with  an  alkali  free 
from  carbonate.  Commercial  sodium  hydroxide  always 
contains  carbonate,  and  even  when  a  pure  solution  is  prepared 
from  sodium  and  water  free  from  carbon  dioxide,  it  soon 
becomes  contaminated  with  sodium  carbonate  by  exposure 
to  air.  Barium  and  calcium  hydroxide  solutions,  on  the 
other  hand,  are  easily  obtained  free  from  carbonate,  since 
the  carbonates  are  nearly  insoluble  in  water  (and  still  less 
soluble  in  solutions  of  the  hydroxides),  and  if  protected 
from  carbon  dioxide  they  form  convenient  and  accurate 
standard  solutions  for  use  with  the  above  indicators. 


62 


VOLUMETRIC  ANALYSIS 


Preparation  of  N/20  Baryta  Solution. — Dissolve  about 
30  grams  of  barium  hydroxide  (Ba(OH)2,8H2O)  and  10  grams 
of  barium  chloride  in  400  c.c.  of  boiling  water  contained  in 
a  flask,  then  fit  the  flask  with  a  cork  carrying  a  soda-lime 
tube,  and  set  aside  until  cold.  The  excess  of  baryta 
crystallises,  and  a  clear  saturated  solution,  which  is  about 
0-3  normal,  is  obtained. 

The  standard  baryta  solution  must  be  kept  in  a  bottle 
which  is  permanently  connected  with  a  burette,  atmospheric 
carbon  dioxide  being  excluded  by  means  of  the  soda-lime 

tubes  A  and  B  (see  Fig.  23). 
Pour  about  2  litres  of  water  into 
the  bottle  (a  Winchester  quart), 
and  connect  it  with  the  burette 
as  shown.  Close  the  burette 
tap,  attach  the  tube  A  to  a 
water-pump,  and  draw  a  current 
of  air  free  from  carbon  dioxide 
through  the  burette  and  bottle 
for  ten  minutes.  (As  the  soda- 
lime  tube  B  is  small,  more  effi- 
cient purification  of  a  rapid  air- 
current  is  secured  by  attaching 
temporarily  a  large  soda-lime 
tower  to  the  tube  D.)  Then  lift 
the  cork  C,  carefully  decant  the 
cold  baryta  solution  into  the 
bottle  and  replace  the  cork.  Mix 
the  contents  of  the  bottle  by 
drawing  a  current  of  CO2-free 
air  through  the  solution,  and 
fill  the  burette  by  opening  the  clip  E  and  applying  suction 
at  D.  If  the  solution  is  slightly  turbid  owing  to  a  trace  of 
suspended  barium  carbonate,  allow  it  to  stand  overnight 
before  drawing  it  into  the  burette.  The  solution  should  be 
approximately  0-05  normal,  and  it  may  be  standardised  with 
pure  succinic  acid,  potassium  tetroxalate,  or  decinormal 
hydrochloric  acid,  using  phenolphthalein  as  indicator. 

When  the  burette  is  not  in  use,  it  should  be  kept  filled 
up  above  the  zero  mark,  and  when  used  intermittently  the 


FIG.  23. 


STANDARD  CALCIUM  HYDROXIDE  63 

first  5  or  10  c.c.  run  out  of  the  burette  are  rejected.     If  a 

burette  with  side-tube  is  not  available,  an  ordinary  burette 

fitted  with  a  T-piece,  as  shown  at 

F  in  Fig.  23,  is  equally  convenient. 

The  burette  may  be  fixed  in   an 

ordinary    clamp,    the    Winchester 

resting   on  the  base  of  the  retort 

stand ;  or  it  may  be  attached  to  the  bottle  by  means  of  an 

Ostwald  burette  clamp  (Fig.  24). 


STANDARD  CALCIUM  HYDROXIDE  SOLUTION. 

(7V/25  solution  contains  1-482  grams  Ca(OH).2per  litre.} 

A  saturated  solution  of  calcium  hydroxide  is  about 
0-04  normal.  It  is  made  by  shaking  up  excess  of  freshly 
slaked  lime  with  water  in  a  Winchester  quart  bottle,  which 
is  then  set  aside  for  some  days  until  the  solution  has  become 
clear.  The  clear  solution  is  then  syphoned  into  another 
empty  Winchester  similar  to  that  used  for  baryta  solution, 
the  carbon  dioxide  in  the  burette  and  bottle  having  been 
previously  removed  by  means  of  a  current  of  purified  air. 

Calcium  hydroxide  solution  is  standardised  in  the  same 
manner  as  baryta  solution. 


Standard  Potassium  Permanganate  and 
Dichromate 

DEC1NORMAL  POTASSIUM  PERMANGANATE. 

solution  contains  3.161  grams  KMnO±  per  litre.) 


WEIGH  3-161  grams  of  pure  potassium  permanganate,  wash 
into  a  standard  litre  flask  and  dissolve  in  about  500  c.c.  of 
cold  water.  It  is  inadvisable  to  add  more  water  until  all  the 
salt  has  dissolved,  as  it  is  difficult  to  tell  when  solution  is 
complete  if  the  flask  is  at  once  filled.  Dilute  to  I  litre. 
The  solution  should  not  be  heated  to  dissolve  the  perman- 
ganate, nor  should  the  crystals  be  ground  up  before  solution, 
since  in  both  cases  slight  decomposition  occurs  with  separa- 
tion of  manganese  dioxide,  which  catalytically  decomposes 
more  of  the  permanganate.  If  the  solution  is  to  be  kept  for 
many  months  it  should  be  stored  in  the  dark.  If  any  brown 
sediment  appears  in  a  permanganate  solution,  partial 
decomposition  has  occurred  and  a  fresh  solution  should  be 
prepared. 

On  account  of  its  action  on  rubber  and  other  organic 
materials,  potassium  permanganate  should  be  kept  in  a 
bottle  with  a  glass  stopper,  and  the  burette  used  for  the 
titration  must  have  a  glass  tap.  With  permanganate 
solutions  the  burette  readings  should  be  taken  at  the  top  of 
the  meniscus,  since  it  is  not  possible  to  see  the  lowest 
portion  on  account  of  the  colour  of  the  solution. 

In  all  titrations  with  potassium  permanganate  sulphuric 
acid  must  be  added.  If  a  brown  precipitate  appears  during 
the  titration,  the  results  will  be  inaccurate,  and  the  titration 
should  be  repeated  with  addition  of  more  sulphuric  acid. 


STANDARD  POTASSIUM  PERMANGANATE          65 

Permanganate  in  presence  of  acid  oxidises  ferrous  sulphate 
as  follows  :  — 

2KMn04  +  4H,S04  =  2KHSO4  +  2MnSO4  +  50  +  3H2O 
ioFeSO4  +  50  +  5H2SO4  =  5Fe2(SO4)3  +  5H2O, 

or,  combining  these, 

2KMnO4  +  9H2SO4+  ioFeSO4 

=  5Fe2(SO4)3  +  2KHSO4  +  2MnSO4  +  8H2O. 

Two  molecules  of  permanganate  provide  ten  equivalents 
of  oxygen,  and  a  decinormal  solution  contains  one-fiftieth  of 
the  gram-molecular  weight  per  litre.  With  pure  perman- 
ganate and  water  free  from  organic  matter,  the  concentration 
of  the  solution  as  calculated  from  the  weight  taken  should  be 
correct.  It  is  advisable,  however,  to  check  this  by  titration 
against  potassium  tetroxalate  or  ferrous  ammonium  sulphate, 
two  salts  which  are  readily  obtained  in  a  pure  state. 

Titrations  with  Permanganate  in  presence  of  Hydro- 
chloric Acid.  —  Hydrochloric  acid  in  presence  of  certain  other 
substances,  such  as  ferric  salts,  interacts  with  permanganate 
and  therefore  introduces  an  error.  This  action  may  be 
altogether  prevented  by  the  addition  of  phosphoric  acid.  In 
the  titration  of  iron  with  permanganate,  the  amount  of 
phosphoric  acid  added  must  be  sufficient  to  keep  the 
solution  colourless  until  the  end-point  is  reached.  If  any 
brown  or  yellow  colour  is  noticed,  insufficient  phosphoric  acid 
has  been  added  and  the  titration  will  be  inaccurate. 

Standardisation  with  Ferrous  Ammonium  Sulphate.  — 
Ferrous  ammonium  sulphate  has  the  formula  FeSO4, 
(NH4)2SO4,6H.2O.  Weigh  exactly  i-o  to  i-i  gram,  wash  into 
a  conical  flask  containing  about  25  c.c.  of  dilute  sulphuric 
acid,  and  run  in  the  permanganate  solution  until  a  faint 
permanent  pink  coloration  is  obtained. 

Calculation.  —  The  oxidation  of  a  ferrous  to  a  ferric 
compound  may  be  represented  in  its  simplest  form  by  the 
equation 


It  is  evident  that  55-84  grams  of  iron  are  oxidised  by  8  grams 
(i  gram-equivalent)  of  oxygen.  But  I  gram-molecule 
(392'3  grams)  of  ferrous  ammonium  sulphate  contains  55-84 

E 


66  VOLUMETRIC  ANALYSIS 

grams  of  iron,  and  is,  therefore,  oxidised  by  I  gram-equivalent 
of  oxygen,  i.e.,  by  I  litre  of  normal  permanganate. 

If  26-80  c.c.  of  the  permanganate  oxidises  1-052  gram  of 
ferrous  ammonium  sulphate, 

r.         .n       .,.      1-052x1000 
i  litre  will  oxidise  —  "TQ  --  grams. 


But  I  litre  of  normal  permanganate  will  oxidise  392-3  grams. 
Therefore  the  permanganate  solution  is 

I-052X  1000 


26.80  x  39^-3 
Two  experiments  are  sufficient  if  the  results  are  concordant. 

Standardisation     with     Potassium     Tetroxalate.  —  The 

accuracy  of  the  potassium  permanganate  may  be  checked  by 
titration  against  potassium  tetroxalate.  Weigh  accurately 
about  0-16  gram  of  the  salt,  wash  it  into  a  conical  flask, 
and  add  about  25  c.c.  of  dilute  sulphuric  acid.  Warm  the 
solution  to  about  70°  and  titrate  by  running  in  the  potassium 
permanganate  solution  until  the  colour  is  no  longer  dis- 
charged. A  faint  permanent  pink  colour  marks  the  end- 
point.  The  permanganate  should  be  run  in  slowly.  The 
formation  of  a  brown  precipitate  or  coloration  indicates  that 
the  solution  is  too  cold  or  that  insufficient  sulphuric  acid  has 
been  added. 

The  sulphuric  acid  interacts  with  the  potassium  tetrox- 
alate to  yield  oxalic  acid, 

KHC204,  H2C204;  2  H20  +  H2S04  =  2  H2C2O4  +  K  HSO4  +  2  H2O. 

The  oxalic  acid  is  then  oxidised  by  the  potassium  perman- 
ganate according  to  the  equation 

2KMnO4  +  5H2C2O4  +  4H2SO4 

=  2KHSO4  +  2MnSO4  +  ioCO2  +  8H2O. 

If  the  solution  is  too  cold  or  if  insufficient  acid  is  present, 
secondary  reactions  take  place. 

It  may  be  seen  from  the  first  of  the  above  equations  that 
I  molecule  of  the  tetroxalate  is  equivalent  to  2  molecules  of 
oxalic  acid,  and  therefore  requires  2  atoms  (or  four  equiva- 
lents) of  oxygen  to  oxidise  it,  thus  :  — 

2K,C  A  +  20  =  2H20  +  4C02. 


STANDARD  POTASSIUM  PERMANGANA1E          67 

One  litre  of  normal  permanganate  will  therefore  oxidise 
one-fourth  of  the  molecular  weight,  z>.,  63-55  grams,  of  the 
tetroxalate. 

Oxalic  acid,  H2C2O4,2H2O,  may  be  used  instead  of 
potassium  tetroxalate  for  the  standardisation,  but  it  effloresces 
more  readily  and  is  not  so  easily  obtained  in  a  pure  state. 

ANALYSES    INVOLVING    THE    USE    OP    STANDARD 
PERMANGANATE. 

One  of  the  most  important  determinations  that  can  be 
made  by  means  of  a  standard  solution  of  potassium  per- 
manganate is  that  of  iron.  Iron  can  be  determined,  however, 
equally  well  with  standard  potassium  dichromate,  and  all  the 
examples  of  analyses  involving  the  determination  of  iron  are 
given  after  the  preparation  and  use  of  a  standard  dichromate 
solution  has  been  described  (see  p.  74), 

Oxalic  Acid  and  Oxalates. 

To  a  weighed  quantity  (or  a  measured  volume)  add 
excess  of  dilute  sulphuric  acid,  warm,  and  titrate  the  oxalic 
acid.  The  procedure  is  described  under  the  standardisation 
of  permanganate  with  potassium  tetroxalate. 

Peroxides. 

The  peroxide  is  boiled  with  excess  of  oxalic  acid  (or  a 
soluble  oxalate)  and  dilute  sulphuric  acid  until  the  reaction 
is  complete.  Part  of  the  oxalic  acid  is  oxidised  by  the 
peroxide  and  the  residual  portion  is  determined  by  titration 
with  standard  permanganate.  The  equation  in  the  case  of 
manganese  dioxide  is 

Mn02  +  H2C204  +  H2S04  =  MnSO4  +  2H2O  +  2CO2. 

Valuation  of  Manganese  Dioxide  (Pyrolusite). — Prepare 
an  approximately  0-25  normal  solution  of  oxalic  acid. 
Weigh  exactly  about  0-4  gram  of  finely  powdered  pyrolusite 
in  a  small  weighing  tube  (a  piece  of  glass  tubing,  \  inch  long, 
closed  at  one  end)  and  drop  the  tube  and  contents  into  a 
flask  (about  250  c.c.).  Measure  50  c.c.  of  the  oxalic  acid  into 


68  VOLUMETRIC  ANALYSIS 

the  flask,  add  25  c.c.  of  dilute  sulphuric  acid,  place  a  small 
funnel  in  the  mouth  of  the  flask,  and  boil  gently  until  no  black 
particles  remain  undissolved.  (A  small  residue  of  silica  is 
usually  present.)  Titrate  the  hot  solution  with  standard 
permanganate.  Also  titrate  10  c.c.  of  the  oxalic  acid  solution 
with  the  permanganate. 

Calculation. — 0-4084  gram  of  pyrolusite  was  boiled  with 
50  c.c.  of  an  oxalic  acid  solution.  The  residual  oxalic  acid 
required  24-60  c.c.  of  0-1025  N  permanganate.  Also,  10  c.c. 
of  the  oxalic  acid  required  21-20  c.c.  of  the  permanganate, 
and  therefore  50  c.c.  require  106-0  c.c.  Therefore  the 
MnO2  is  equivalent  to  106-0  —  24-60  =  81-40  c.c.  of  0-1025  N 
permanganate. 

From  the  equation  it  may  be  seen  that  the  equivalent  of 
MnO2  is  43-46.  One  c.c.  of  normal  permanganate  therefore 
corresponds  to  0-04346  gram  MnO2.  The  percentage  of 
MnO2  in  the  sample  is  therefore 

81-4x0-1025x0-04346x100       00 

=  oo-Q. 

0-4084 

In  place  of  oxalic  acid,  ferrous  sulphate  may  be  used.  In 
this  case,  the  flask  must  be  provided  with  a  Bunsen  valve 
(p-  75)>  or  w*tn  a  delivery  tube  dipping  into  sodium 
carbonate  solution  (p.  75),  in  order  to  protect  the  ferrous 
sulphate  from  atmospheric  oxidation. 


Nitrite. 

Nitrous  acid  is  oxidised  quantitatively  by  potassium 
permanganate  to  nitric  acid.  If  the  solution  is  made 
strongly  acid  prior  to  the  titration,  there  is  danger  of  loss 
of  nitrous  acid  by  volatilisation  ;  this  is  avoided  by  adopting 
the  following  procedure  : — 

Dissolve  a  weighed  quantity  of  the  nitrite  in  cold  water, 
and  add  decinormal  permanganate  until  the  solution  is 
distinctly  pink.  Add  two  or  three  drops  of  dilute  sulphuric 
acid  and,  immediately  thereafter,  a  known  excess  of 
permanganate.  (An  appreciable  excess  is  necessary,) 
Add  about  20  c.c.  of  dilute  sulphuric  acid,  boil  for  a  few 


STANDARD  POTASSIUM  PERMANGANATE          69 

minutes,  and  titrate  the  residual  permanganate  with  a 
decinormal  oxalic  acid  solution. 

Exercise. — Determine  the  percentage  of  the  pure  salt  in 
a  commercial  sample  of  potassium  nitrite.  Weigh  accurately 
i  to  1-2  gram,  dissolve  in  cold  water,  and  dilute  to  250  c.c. 
Use  25  c.c.  for  each  titration. 

Calcium. 

The  calcium  is  precipitated  as  calcium  oxalate.  The 
washed  precipitate  is  dissolved  in  sulphuric  acid  and  the 
solution  is  titrated  with  standard  permanganate. 

From  the  equation 

CaC2O4  +  H2SO4  +  (O)  =  CaSO4  +  2CO2  +  H2O 

it  may  be  seen  that  i  gram-molecule  of  CaC2O4  (containing 
40-07  grams  Ca)  requires  2  gram-equivalents  of  oxygen. 
The  gram-equivalent  of  calcium  is  therefore  20-03,  an^  l  c-c- 
normal  permanganate  corresponds  to  0-02003  gram  Ca. 

Exercise. — Weigh  accurately  about  0-15  gram  of  powdered 
calcspar.  Transfer  to  a  300  c.c.  beaker,  add  10  c.c.  of  water 
and  5  c.c.  of  dilute  hydrochloric  acid,  and  cover  the  beaker 
with  a  clock-glass.  Warm  until  the  calcspar  has  dissolved, 
then  dilute  with  a  little  water  and  boil  the  solution  for  a  few 
minutes  in  order  to  free  it  from  carbon  dioxide.  Rinse  the 
clock-glass  into  the  beaker  and  add  a  few  drops  of  methyl 
orange  and  then  ammonia  until  the  solution  is  nearly  but 
not  quite  alkaline.  Dilute  the  solution  to  about  150  c.c., 
heat  to  boiling,  and  precipitate  the  calcium  by  slowly  adding 
a  boiling  solution  of  ammonium  oxalate.  (Use  a  freshly 
prepared  solution,  about  2  per  cent.)  Now  make  alkaline 
with  ammonia  and  boil  for  a  few  minutes,  stirring  in  order  to 
avoid  "  bumping."  Allow  the  precipitate  to  settle,  and  then 
make  sure  that  precipitation  is  complete  by  adding  a  few 
drops  more  of  the  reagent.  Place  the  beaker  on  the  steam- 
bath  for  one  hour. 

Decant  the  supernatant  liquid  through  a  9  cm.  paper, 
wash  the  precipitate  once  by  decantation,  and  then  trans- 
fer it  to  the  filter  (see  p.  25).  Wash  the  precipitate  and 
filter  paper  with  hot  water  containing  a  little  ammonia, 
until  the  washings  give  no  opalescence  with  nitric  acid  and 


70  VOLUMETRIC  ANALYSIS 

silver  nitrate.  (Always  rinse  the  end  of  the  funnel  stem 
before  collecting  a  portion  of  the  washings  —  about  5  c.c.  — 
for  a  test  like  this.) 

Now  place  under  the  funnel  the  beaker  in  which  the 
precipitation  was  made,  pierce  the  apex  of  the  filter  paper 
with  a  pointed  glass  rod,  and  wash  the  precipitate  into  the 
beaker.  Pour  about  25  c.c.  of  hot  dilute  sulphuric  acid  into 
the  filter,  taking  care  that  the  acid  comes  into  contact  with 
every  part  of  the  paper  and  that  some  of  it  is  poured  behind 
the  double  fold  of  the  paper,  in  case  any  of  the  calcium 
oxalate  should  have  lodged  there.  Then  wash  the  paper 
thoroughly  with  hot  water.  Heat  the  calcium  oxalate 
solution  to  about  70°,  and  titrate  with  standard  perman- 
ganate. 

Calculate  the  percentage  of  calcium  in  the  calcspar. 

Nitrate. 

When  a  solution  of  ferrous  sulphate  is  boiled  with  a 
nitrate  and  excess  of  sulphuric  acid,  the  ferrous  sulphate  is 
oxidised  and  nitric  oxide  is  set  free. 


=  3Fe2(S04)3  +  2NO  +  2  KHS04  +  4H2O. 

If,  therefore,  a  known  quantity  of  ferrous  sulphate  is 
taken,  the  amount  of  nitrate  present  can  be  calculated  from 
the  amount  of  ferrous  sulphate  which  becomes  oxidised  in 
the  process.  Air  must  be  carefully  excluded  during  the 
process,  more  especially  as  nitric  oxide  and  oxygen  form 
nitrogen  peroxide  which  would  then  oxidise  more  of  the 
ferrous  sulphate.  The  air  is  accordingly  displaced  by  a 
current  of  carbon  dioxide. 

The  mixture  must  contain  from  35  to  40  per  cent,  by 
volume,  of  concentrated  sulphuric  acid. 

The  apparatus  is  shown  in  Fig.  25.  It  consists  of  a  400 
c.c.  conical  flask,  fitted  with  a  reflux  condenser,  and  provided 
with  a  tube  A,  through  which  the  carbon  dioxide  enters. 
A  U-tube,  containing  a  little  water,  is  attached  to  the  top 
of  the  condenser.  The  carbon  dioxide  is  supplied  from  a 
Kipp  generator.  If  the  generator  has  been  freshly  charged, 
the  air  must  be  carefully  displaced  before  using  the  gas. 


STANDARD  POTASSIUM  DICHROMATE 


71 


Procedure. — Dissolve  about  45  grams  of  ferrous  sulphate 
in  water,  add  50  c.c.  of  dilute  sulphuric  acid,  and  dilute  the 
solution  to  I  litre.  Titrate  25  c.c.  of  the  solution  with 
decinormal  permanganate. 

Weigh  accurately  about  I  gram  of  the  nitrate, 
dissolve  in  water,  and  dilute  the  solution  to 
250  c.c.  in  a  standard  flask.  Measure  25  c.c. 
of  the  solution  into  the  conical  flask,  and  add 
25  c.c.  of  the  ferrous  sulphate  solution. 
Connect  the  flask  with  the  condenser,  and  pass 
a  rapid  current  of  carbon  dioxide  through  the 
apparatus  for  five  minutes.  Then,  without  in- 
terrupting the  current  of  carbon  dioxide, 
immerse  the  flask  in  cold  water,  remove  the 
U  -  tube,  and  slowly  introduce,  through  the 
condenser,  30  c.c.  of  concentrated  sulphuric  acid. 
Replace  the  U-tube,  and  reduce  the  amount  of 
carbon  dioxide  entering  the  flask  until  about 
one  bubble  of  gas  per  second  passes  through 
the  water  in  the  U-tube.  Boil  the  contents 
of  the  flask  for  ten  minutes.  Increase  the  rate 
of  the  carbon  dioxide  again,  and  cool  the  solu- 
tion by  immersing  the  flask  in  water.  Detach 
the  flask  and  rinse  the  carbon  dioxide  inlet  tube 
into  it.  Dilute  the  solution  to  about  150  c.c., 
and  titrate  the  residual  ferrous  sulphate  with  decinormal 
permanganate. 

Calculate  the  percentage  of  NO3  in  the  substance. 

One  c.c.  of  normal  permanganate  corresponds  to  0-02067 
gram  NO3. 

Exercise. — Determine  the  percentage  of  NO3  in  a  sample 
of  potassium  nitrate. 


FIG.  25. 


DECINORMAL    POTASSIUM    DICHROMATE. 

(N\\o>  solution  contains  4-903  grams  K^Cr^O^ per  litre.) 

Weigh  accurately  about  4-9  grams  of  pure  potassium 
dichromate.  Transfer  to  a  standard  litre  flask,  dissolve  in 
water,  and  dilute  the  solution  to  i  litre. 


72  VOLUMETRIC  ANALYSIS 

The  interaction  between  potassium  dichromate  and  ferrous 
sulphate  in  presence  of  acid  is  as  follows  :— 

K2Cr207  +  sH2S04  -  2KHSO4  +  Cr,(SO4)3  +  4H0O  +  30' 
6FeS04  +  3H2S04  +  3O  =  3Fe2(SO4)3  +  3H2O, 

or,  combining  these  equations, 

K2Cr2O7  +  6FeSO4  +  8H2SO4 

m  2KHS04  +  Cr2(S04)3  +  3Fe2(SO4)3  +  yH,O. 

One  molecule  of  the  dichromate  thus  provides  six  equivalents 
of  available  oxygen,  and  a  decinormal  solution  therefore 
contains  one-sixtieth  of  the  gram-molecular  weight  per  litre. 
If  the  solution  is  prepared  from  pure  potassium  dichromate, 
the  concentration  should  correspond  exactly  to  the  weight  of 
the  salt  taken.  Standard  potassium  dichromate  is  used  only 
for  the  determination  of  iron,  and  I  litre  of  a  decinormal 
solution  will  oxidise  5-584  grams. 

Dichromate,  unlike  permanganate,  is  unaffected  by 
moderate  quantities  of  hydrochloric  acid,  and  it  is  therefore 
suitable  for  the  titration  of  iron  when  stannous  chloride  has 
been  used  as  reducing  agent.  It  has  no  action  on  rubber,  and 
may  be  measured  in  a  burette  closed  with  rubber  tubing  and 
a  pinchcock,  instead  of  a  glass  tap.  The  solution  is  quite 
stable. 

Titration  with  Dichromate. — The  solution  to  be  titrated 
(which  should  be  placed  in  a  beaker,  a  flask  is  not  convenient) 
must  contain  considerable  sulphuric  or  hydrochloric  acid  ; 
add,  therefore,  about  25  c.c.  of  dilute  (2N)  acid,  unless 
the  solution  is  known  to  contain  a  corresponding  quantity, 
and  then  run  in  the  dichromate  solution  from  a  burette 
until  all  the  iron  is  oxidised. 

The  green  chromic  salt  formed  during  the  titration 
obscures  the  colour  change  at  the  end-point,  and  few  are  able 
to  detect  it  by  eyesight.  It  is  therefore  usual  to  determine 
the  end-point  by  means  of  potassium  ferricyanide,  which  is 
used  as  an  "external"  indicator.  The  ferricyanide  solution 
must  be  freshly  prepared  and  very  dilute  (about  o  •  I  per  cent.}, 
and  must  contain  no  ferrocyanide.  To  prepare  it,  take  a  crystal 
of  potassium  ferricyanide  weighing  slightly  more  than  0-5 
gram,  rinse  it  several  times  with  small  quantities  of  cold 
water  (in  order  to  remove  superficial  ferrocyanide),  and 


STANDARD  POTASSIUM  DICHROMATE  73 

dissolve  in  50  c.c.  of  water.  Dilute  5  c.c.  of  this  solution  to 
50  c.c.  The  solution  decomposes  somewhat  rapidly,  and  a 
fresh  one  must  be  made  from  the  solid  for  each  set  of 
titrations.1 

The  titration  is  then  carried  out  as  follows  : — Run  in  the 
dichromate  solution  slowly  from  a  burette  whilst  stirring  the 
ferrous  solution,  and  from  time  to  time  place  a  drop  of  the 
latter  on  a  white  porcelain  tile  and  touch  it  with  a  drop  of 
the  ferricyanide  indicator ;  the  ferricyanide  should  be  placed 
alongside  of  the  other  drop  so  that  the  two  drops  coalesce. 
(A  separate  glass  rod  must  be  used  for  the  ferricyanide,  and 
after  each  test  it  should  be  placed  in  a  beaker  of  water  in 
order  to  rinse  it.)  If  the  solution  still  contains  considerable 
ferrous  salt,  a  blue  coloration  is  obtained ;  as  the  amount  of 
ferrous  salt  diminishes,  the  blue  coloration  becomes  less 
pronounced  until  only  a  faint  green  tint  is  seen,  and  finally, 
when  no  trace  of  this  can  be  detected  after  waiting  for  thirty 
seconds,  no  ferrous  salt  remains  and  the  titration  is  finished. 

If  many  drops  are  removed  at  an  early  stage  of  the 
titration,  the  accuracy  is  impaired,  and  a  second  titration 
must  then  be  made  in  which  almost  the  whole  amount  of 
the  dichromate  is  added  before  any  drops  are  removed  for 
testing.  The  error  due  to  the  removal  of  drops  is  then 
negligible.  If  the  ferricyanide  contains  ferrocyanide,  it  is 
impossible  to  obtain  a  sharp  end-point,  since  ferric  salts  give 
a  blue  coloration  with  ferrocyanide. 

Standardisation. — If  there  is  any  doubt  as  to  the  purity 
of  the  potassium  dichromate,  the  solution  must  be  standard- 
ised against  ferrous  ammonium  sulphate  or  iron  of  known 
purity. 

(i)  Weigh  accurately  about  I  gram  of  ferrous  ammonium 
sulphate,  wash  into  a  beaker  containing  about  25  c.c.  of  dilute 
sulphuric  acid,  dilute  to  about  100  c.c.,  and  titrate  with  the 
dichromate  solution  in  the  manner  described  in  the  preceding 
paragraph.  As  the  concentration  of  the  dichromate  is 
approximately  known,  no  drops  of  the  ferrous  solution  need 
be  removed  for  testing  until  the  titration  is  almost  complete. 

1  After  a  little  experience  it  is  easy  to  prepare  a  solution  of  a  suitable 
concentration  by  carefully  rinsing  a  minute  crystal  of  ferricyanide  and 
dissolving  it  in  a  test-tube  of  water. 


74  VOLUMETRIC  ANALYSIS 

An  alternative  method  is  as  follows : — Weigh  (to  the 
nearest  centigram)  about  10  grams  of  the  ferrous  salt,  transfer 
to  a  standard  250  c.c.  flask  containing  about  50  cc.  of  dilute 
sulphuric  acid,  and,  after  the  salt  has  dissolved,  make  up  to 
the  mark.  Take  25  c.c,  add  about  20  c.c.  of  dilute  sulphuric 
acid,  dilute  to  about  100  c.c.,  and  titrate. 

(2)  If  iron  wire  is  to  be  used  as  standard,  a  weighed 
quantity  is  dissolved  in  acid  by  the  method  given  below,  in 
order  that  no  oxidation  of  the  ferrous  salt  may  take  place 
prior  to  titration.  The  "  apparent "  percentage  of  iron  in  the 
wire  must  be  known  (see  below). 

ANALYSES    INVOLVING    THE    USE    OP    STANDARD 
PERMANGANATE    OR   DIOHROMATE    SOLUTIONS. 

The  determination  of  iron  may  be  made  with  either  a 
standard  permanganate  or  a  standard  dichromate  solution, 
the  choice  being  in  most  cases  a  matter  of  convenience  or 
personal  preference. 

Iron  in  Iron  Wire. 

Commercial  iron  is  not  chemically  pure,  although  the 
amount  of  impurity  in  some  varieties  of  iron  is  very  small. 
As  determined  by  volumetric  methods,  commercial  iron 
sometimes  appears  to  contain  more  than  100  per  cent,  of  pure 
iron.  This  is  due  to  the  presence  of  impurities  (carbides, 
etc),  which  have  a  greater  reducing  action  when  dissolved  in 
acid  than  an  equal  weight  of  pure  iron. 

The  determination  of  the  "  apparent  "  percentage  of  iron 
in  iron  wire  is  made  as  follows : — Remove  any  trace  of  rust 
from  some  fine  piano  wire  by  means  of  emery  cloth,  and  then 
clean  the  wire  with  filter  paper.  Weigh  accurately  about  0-6 
gram  of  the  wire  and  place  it  in  a  200  c.c.  flask  fitted  with  a 
rubber  stopper  and  bent  tube  (Fig.  26).  The  bent  tube 
should  dip  into  a  sodium  carbonate  solution  contained  in  a 
beaker.  Pour  about  10  c.c.  of  sodium  carbonate  solution  into 
the  flask  in  order  that  carbon  dioxide  will  fill  the  flask  when 
acid  is  added. 

Dilute  10  c.c.  of  concentrated  sulphuric  acid  by  pouring  it 
into  20  c.c.  of  water,  and  pour  this  mixture  into  the  flask. 


IRON  IN  IRON  WIRE 


75 


Replace  the  cork  securely,  and  warm  gently  until  all  the  iron 
has  dissolved.  (Minute  particles  of  carbon  sometimes  remain 
undissolved.)  Allow  the  solution  to  cool ;  as  it  does  so,  some 
of  the  sodium  carbonate  will  pass  into  the  flask  and,  by  inter- 
action with  the  acid,  yield  carbon  dioxide.  The  solution  is 
thereby  protected  from  oxidation  by  the  atmosphere.  When 
nearly  cold,  detach  the  flask  from  the  cork  and  tube  and  hold 
it  under  the  tap  until  cold. 

Pour  the  solution  into  a  standard   100  c.c.  flask,  wash  the 
original  flask  several  times,  and  dilute  the  solution  and  wash- 


Fic.  26. 


FIG.  27. 


ings  to  the  graduation  mark.  Mix  the  contents  of  the  flask 
thoroughly,  and  titrate  portions  of  25  c.c.  with  standard 
permanganate  or  dichromate. 

Alternative  method. — The  solution  of  the  iron  wire 
may  be  prepared  in  a  flask  fitted  with  a  Bunsen  valve 
(Fig.  27).  This  consists  of  a  narrow  rubber  tube  closed 
with  a  short  piece  of  glass  rod ;  a  longitudinal  slit  in 
the  rubber  allows  gas  to  escape  outwards,  but  prevents  any 
ingress  of  air.  The  procedure  is  otherwise  identical  with 
that  already  described. 

Calculate  the  percentage  of  iron  in  the  wire,  assuming 
that  the  reducing  action  of  the  solution  is  due  only  to  iron. 

Note  on  the  Oxidation  of  Ferrous  Salts  by  Atmospheric 

Oxygen. 

The  precautions  taken  to  prevent  oxidation  in  the  above 
experiment  may  suggest  that  ferrous  salts  are  more  readily 


76  VOLUMETRIC  ANALYSIS 

oxidised  than  is  really  the  case.  Dry  ferrous  salts  do  not 
become  oxidised  in  pure  air.  A  solution  of  ferrous  sulphate 
containing  free  sulphuric  acid  is  not  oxidised  at  room 
temperature  even  if  air  or  oxygen  is  blown  through  it  for 
several  hours.  In  hot  acid  solution  oxidation  occurs  to  an 
appreciable  extent,  although  slowly.  If  no  acid  except  that 
formed  by  hydrolysis  is  present,  oxidation  proceeds  even  in 
cold  solution.  Ferrous  hydroxide  rapidly  becomes  oxidised 
on  exposure  to  the  atmosphere. 

Iron  in  Ferrous  and  Ferric  Compounds. 

If  the  iron  is  wholly  present  in  the  ferrous  condition,  it  is 
determined  by  direct  titration  as  described  under  standardisa- 
tion of  permanganate  or  dichromate,  by  means  of  ferrous 
ammonium  sulphate. 

If  the  iron  is  present,  wholly  or  partly,  as  ferric  salt,  it 
must  be  reduced  to  the  ferrous  state  before  titration.  The 
common  methods  of  reduction  are  : — 

(1)  With  zinc  and  acid   (this  method   can  only  be  used 

in   conjunction  with   permanganate,  as   zinc   salts 
interfere  with  the  dichromate  titration) ; 

(2)  With  sulphur  dioxide  ; 

(3)  With  hydrogen  sulphide ; 

(4)  With  stannous  chloride  (this  method  can  be  used  only 

in   conjunction   with    the    dichromate    method    of 
titration). 

As  the  reducing  agents  themselves  also  reduce  permanganate 
or  dichromate  solutions,  any  excess  of  the  reducing  agent 
must  be  removed  before  the  titration. 

Exercise. — Determine  the  percentage  of  iron  in  iron 
alum  [potassium-  or  ammonium-ferric  sulphate,  Fe2(SO4)3, 
(NH4)2SO4,24H2O],  using  the  various  methods  of  reduction 
described  below,  and  compare  the  results. 

Dissolve  about  1 2  grams  (weighed  to  the  nearest  centi- 
gram) of  iron  alum  in  water  to  which  25  c.c.  of  dilute 
sulphuric  acid  has  been  added.  Dilute  in  a  standard  flask 
to  250  c.c.,  and  use  25  c.c.  for  each  determination. 

Reduction  -with  Zinc. — To  25  c.c.  of  the  iron  alum 
solution  contained  in  a  conical  flask,  add  about  20  c.c.  of 


IRON  IN  FERRIC  COMPOUNDS  77 

dilute  sulphuric  acid  and  a  piece  of  zinc  rod  (free  from 
iron)  about  an  inch  long.  Warm  gently  and  allow  the 
reaction  to  continue  until  the  solution  appears  quite  colour- 
less, and  then  test  for  ferric  iron  : — Place  a  drop  of  potassium 
thiocyanate  solution  on  a  white  porcelain  surface,  such  as  a 
crucible  lid,  and  then  bring  into  contact  with  the  drop  a 
trace  of  the  iron  solution  which  has  been  withdrawn  from  the 
flask  by  means  of  a  thin  glass  rod  or  a  capillary  tube ;  a  red 
coloration — it  may  be  only  a  mere  tinge — indicates  that 
reduction  is  incomplete,  t\e.t  that  ferric  salt  is  still  present. 

After  reduction  is  complete,  filter  through  a  small  plug  of 
glass  wool  into  a  flask  containing  25  c.c.  of  dilute  sulphuric 
acid.  Rinse  the  original  flask  and  undissolved  zinc  several 
times  with  dilute  sulphuric  acid,  pouring  this  also  through  the 
filter,  and  wash  the  latter  carefully  with  water.  Titrate  the 
solution  with  standard  permanganate. 

Reduction  with  Zinc  Dust. — The  rate  of  reduction  of 
ferric  salts  by  means  of  zinc  depends  mainly  on  the  surface 
of  zinc  exposed  to  the  solution.  The  concentration  of  acid 
in  the  solution  should  be  just  sufficient  to  prevent  precipita- 
tion of  basic  salts.  With  the  following  method,  reduction  is 
complete  in  a  few  minutes. 

To  a  measured  volume  of  the  ferric  solution  contained  in 
a  boiling  tube,  add  about  I  c.c.  of  dilute  sulphuric  acid  (25  c.c. 
of  the  iron  alum  solution  prepared  as  above  already  contains 
this  amount) ;  if  the  solution  contains  excess  of  acid,  first 
neutralise  it  by  adding  ammonia  until  a  slight  precipitate 
appears,  and  then  add  I  c.c.  of  acid.  Drop  into  the  boiling- 
tube  about  0-5  gram  of  zinc  dust  (about  as  much  as  will  lie 
on  a  sixpence).  Heat  the  mixture  to  the  boiling  point,  boil 
for  one  minute,  and  then  pour  through  a  filter  which  has 
been  covered  with  a  layer  of  zinc  dust,  receiving  the  filtrate 
in  a  flask  containing  25  c.c.  of  dilute  sulphuric  acid. 

A  small  Biichner  funnel  or  a  Gooch  crucible  (the  perfora- 
tions in  either  case  being  covered  with  a  piece  of  filter  paper) 
is  very  suitable,  but  an  ordinary  funnel  and  paper  will  serve. 
Cover  the  filter  with  a  layer  of  zinc  dust  -|-  to  \  inch  deep, 
wash  with  a  little  very  dilute  sulphuric  acid,  and  use  this 
filter  without  further  treatment  for  three  or  four  experiments. 
It  is  preferable  to  filter  with  slight  suction,  but  the  funnel- 


78  VOLUMETRIC  ANALYSIS 

stem  must  be  long  enough  to  prevent  loss  of  liquid  down  the 
side-tube  leading  to  the  filter-pump. 

Wash  the  boiling-tube  and  filter  at  least  four  times  with 
very  dilute  sulphuric  acid  (about  I  c.c.  of  ordinary  dilute 
acid  in  20  c.c.  of  water),  which  should  be  warmed  in  the  tube 
before  pouring  through  the  filter.  Very  thorough  washing  is 
necessary  to  remove  all  the  iron  from  the  zinc  dust. 

Titrate  the  solution  in  the  filter-flask  with  standard 
permanganate. 

If  the  zinc  is  not  free  from  iron,  a  correction  must  be 
applied  as  follows : — Use  a  known  weight  of  the  zinc  (say 
5  grams)  for  the  reduction  of  the  ferric  solution,  and  allow 
the  reaction  to  continue  until  all  the  zinc  has  dissolved. 
(An  insoluble  impurity,  chiefly  lead,  nearly  always  remains.) 
Filter  and  titrate  as  already  described.  Then,  in  order  to 
determine  the  amount  of  jron  in  the  zinc,  dissolve  5  grams 
in  dilute  sulphuric  acid  (10  c.c.  concentrated  acid  mixed  with 
30  c.c.  of  water),  filter  the  solution,  and  titrate  with  per- 
manganate. Note  the  amount  of  permanganate  required 
to  give  the  usual  pink  colour,  and  deduct  this  amount  from 
the  volume  of  permanganate  used  in  the  titration  of  the 
iron  solution. 

Reduction  with  Sulphur  Dioxide. — If  the  solution  is  acid, 
it  must  be  made  nearly  neutral  by  adding  ammonia  until 
a  slight  permanent  precipitate  of  ferric  hydroxide  forms. 
The  reducing  agent  is  then  added,  either  in  the  form  of 
sulphurous  acid  solution  or  a  sulphite,  or  by  passing  a 
current  of  sulphur  dioxide  from  a  syphon  of  the  liquefied 
gas  through  the  solution.  The  solution,  after  the  reducing 
agent  is  added,  must  be  slightly  acid  to  litmus  ;  if  it  is 
alkaline,  no  reduction  takes  place.  The  excess  of  sulphur 
dioxide  is  removed  by  adding  acid  and  passing  a  current 
of  carbon  dioxide  through  the  boiling  solution. 

To  25  c.c.  of  the  iron  alum  solution  add  ammonia  until 
a  slight  permanent  precipitate  forms,  and  then  25  c.c.  of 
sulphurous  acid  solution.  Boil  the  mixture  for  ten  minutes. 
Now  add  about  20  c.c.  of  dilute  sulphuric  acid,  heat  until 
boiling  again,  and  pass  a  fairly  rapid  current  of  carbon 
dioxide  through  the  solution  until  the  sulphur  dioxide  is 
completely  expelled  (about  twenty  minutes.)  Cool  without 


IRON  IN  FERRIC  COMPOUNDS  79 

interrupting  the  gas  current,  and  titrate  with  either  standard 
permanganate  or  dichromate. 

Reduction  with  Hydrogen  Sulphide. — By  passing  a 
current  of  hydrogen  sulphide  through  a  solution  of  a  ferric 
salt,  sulphur  is  precipitated  and  the  ferric  salt  is  completely 
reduced  to  the  ferrous  state.  The  solution  should  contain 
about  2  per  cent,  by  volume  of  concentrated  sulphuric  acid. 
The  excess  of  hydrogen  sulphide  is  removed  by  passing 
a  current  of  carbon  dioxide  through  the  boiling  solution. 

Add  10  c.c.  of  dilute  sulphuric  acid  to  25  c.c.  of  the 
iron  alum  solution  contained  in  a  200  c.c.  flask.  Dilute  to 
about  50  c.c.,  pass  hydrogen  sulphide  into  the  cold  solution 
for  five  minutes,  then  heat  until  boiling,  and  continue  the 
current  of  gas  until  the  precipitated  sulphur  has  coagulated. 
Allow  the  solution  to  cool  somewhat  without  interrupting 
the  gas  current,  and  then  filter  into  another  flask,  rinsing 
the  original  flask  and  washing  the  filter  carefully.  Dilute 
the  solution,  if  necessary,  to  about  100  c.c.,  and  pass  a  fairly 
rapid  current  of  carbon  dioxide  through  the  boiling  solution 
until  hydrogen  sulphide  cannot  be  detected  in  the  escaping 
gas  by  means  of  lead  acetate  paper.1  Cool  the  solution — 
without  interrupting  the  gas  current — by  placing  the  flask  in 
a  basin  of  water.  Rinse  the  gas-delivery  tube  and  remove 
it,  and  then  titrate  the  solution  with  standard  permanganate 
or  dichromate. 

Reduction  with  Stannous  Chloride. — Stannous  chloride 
is  added  to  the  ferric  solution  containing  hydrochloric  acid 
until  the  colour  is  discharged.  The  excess  of  the  stannous 
chloride  is  destroyed  by  adding  mercuric  chloride,  and  the 
solution  is  then  titrated  with  standard  dichromate. 

The  stannous  chloride  solution  may  be  prepared  by 
dissolving  3  grams  of  SnCl2,  2H2O  in  25  c.c.  of  concentrated 
hydrochloric  acid,  and  diluting  to  about  100  c.c. 

To  25  c.c.  of  the  iron  alum  solution  contained  in  a  300  c.c. 
beaker,  add  5  c.c.  of  concentrated  hydrochloric  acid,  heat 
until  boiling,  and  then  run  in  the  stannous  chloride  drop  by 
drop  from  a  burette  until  the  yellow  colour  of  the  solution  is 

1  If  the  solution  is  clear,  ten  to  fifteen  minutes  is  usually  sufficient, 
but  so  long  as  it  is  milky  (on  account  of  sulphur  in  suspension)  a  minute 
trace  of  hydrogen  sulphide  will  always  be  found  in  the  escaping  gas. 


80  VOLUMETRIC  ANALYSIS 

just  discharged.  A  slight  excess  (one  or  two  drops)  of  the 
stannous  chloride  is  essential,  but  a  large  excess  must  be 
carefully  avoided.  Cool  the  solution,  dilute  to  150  c.c.,  and 
then  add  (rapidly,  and  whilst  stirring)  about  10  c.c.  of 
saturated  mercuric  chloride  solution.  A  very  slight,  white 
precipitate  should  form.  If  no  precipitate  appears,  insufficient 
stannous  chlorine  has  been  added,  whilst  a  grey  or  black 
precipitate  shows  that  too  much  stannous  chloride  was  used  ; 
in  either  case  the  experiment  must  be  rejected. 

Titrate  the  turbid  mixture  with  standard  dichromate  (not 
with  permanganate). 

Total  Iron  in  a  Mineral. 

(Hcematite  ;  Magnetite;  Bog  Iron  Ore ;  etc.] 

If  the  ore  is  in  large  pieces,  a  representative  sample  is 
crushed  (without  grinding)  in  a  clean,  steel  percussion 
mortar.  The  coarse  powder  is  then  finely  ground  in  an 
agate  mortar.  Iron  ores  sometimes  dissolve  very  slowly  in 
acid  unless  they  are  reduced  to  an  impalpable  powder. 

Dissolving  the  Ore. — Weigh  accurately  about  I  gram  of 
the  ore,  or  a  larger  quantity  if  the  ore  contains  little  iron.  If 
the  ore  contains  carbonaceous  matter  (as  is  often  the  case), 
weigh  it  in  a  porcelain  crucible,  and  heat  to  dull  redness  for 
ten  minutes. 

Transfer  the  weighed  portion  of  the  ore,  after  ignition,  to 
a  200  c.c.  flask,  and  add  15  c.c.  of  concentrated  hydrochloric 
acid.  Warm  on  the  steam-bath  for  some  time,  and  then 
keep  near  the  boiling  point  of  the  acid  until  the  undissolved 
residue,  if  any,  is  perfectly  white.  (Nothing  is  gained  by  boil- 
ing vigorously — this  merely  weakens  the  acid.) 

If  this  treatment  fails  to  extract  all  the  iron  from  the  ore, 
i.e.,  if  the  residue  is  still  coloured,  add  to  the  hot  liquid  at 
intervals  I  to  2  c.c.  of  stannous  chloride  (avoiding  excess)  ; 
rapid  solution  usually  follows  in  the  case  of  a  haematite  ore. 
If  excess  of  stannous  chloride  is  inadvertently  added,  re- 
oxidise  the  iron  partially  with  a  few  drops  of  potassium 
permanganate. 

When  as  much  as  possible  of  the  ore  has  been  brought 
into  solution,  and  whether  the  residue  is  coloured  or  not, 


IRON  IN  A  MINERAL  81 

dilute  with  a  little  water,  and  filter.  Rinse  the  flask  and  wash 
the  filter  carefully,  first  with  dilute  hydrochloric  acid  and 
then  with  hot  water,  using  as  little  as  possible.  If  the  filtrate 
contains  all  the  iron,  make  it  up  to  100  c.c.  in  a  standard 
flask. 

If  the  insoluble  residue  is  coloured,  and  therefore  probably 
contains  iron  (provided  organic  matter  was  destroyed 
by  ignition),  incinerate  the  filter  paper  together  with  the 
residue  in  a  porcelain  crucible  (p.  120).  Then  add  2  or  3 
grams  of  fused  potassium  hydrogen  sulphate  (pyrosulphate), 
heat  cautiously,  and  keep  the  mixture  fused  until  the  dark 
specks  of  iron  oxide  have  disappeared.  Cool,  place  the 
crucible  in  a  small  porcelain  basin,  and  dissolve  the  solid 
residue  in  the  minimum  quantity  of  dilute  sulphuric  acid. 
Filter  if  necessary  (most  of  the  silica  remains  undissolved), 
add  the  filtrate  and  rinsings  to  the  main  solution  of  the  ore, 
and  dilute  to  100  c.c.  in  a  standard  flask. 

Titration  of  the  Solution. — Use  25  c.c.  of  the  solution  for 
each  titration.  If  stannous  chloride  was  used  in  the  prepara- 
tion of  the  solution,  the  reduction  of  the  ferric  iron  must  also 
be  effected  with  stannous  chloride,  arid  the  solution  titrated 
with  standard  dichromate ;  otherwise  hydrogen  sulphide  or 
sulphur  dioxide  may  be  used,  and  the  titration  made  with 
either  dichromate  or  permanganate. 

Methods  for  Refractory  Oxides  and  Silicates. — The 
acid  treatment  described  above  will  not  always  dissolve 
all  the  iron  in  the  sample.  The  addition,  from  time  to 
time,  of  a  few  crystals  of  potassium  chlorate  to  the  concen- 
trated hydrochloric  acid  promotes  the  solution  of  refractory 
oxides  such  as  magnetite.  The  same  treatment  also  serves 
to  oxidise  any  sulphides  or  carbonaceous  matter  the  sample 
may  contain.  If  potassium  chlorate  has  been  added,  the 
solution  should  then  be  boiled  for  some  time,  or  evaporated 
to  a  small  volume,  in  order  to  free  it  from  chlorine. 

Natural  silicates  and  artificial  silicious  slags  must  be 
decomposed,  as  a  rule,  by  fusing  with  sodium  carbonate,  or 
"  fusion  mixture,"  as  described  under  silica  (p.  206).  If 
only  the  iron  in  the  silicate  is  to  be  determined,  it  is  not 
necessary  to  evaporate  to  dryness  in  order  to  remove  the 
silica.  The  acid  solution  obtained  after  the  fusion  is  treated 

F 


82  VOLUMETRIC  ANALYSIS 

as  follows : — Warm  the  solution  in  a  beaker,  add  a  little 
bromine  water,  and  heat  until  boiling,  in  order  to  oxidise  any 
ferrous  salt.  Precipitate  the  iron  (together  with  aluminium, 
manganese,  etc.)  by  adding  ammonia  in  slight  excess.  Boil 
for  a  minute,  allow  the  precipitate  to  settle,  filter,  transfer 
the  precipitate  to  the  filter,  and  wash  it  thoroughly  with 
hot  water.  Dissolve  the  precipitate  in  hot  hydrochloric  acid 
(equal  volumes  of  concentrated  acid  and  water)  and  wash  the 
filter  carefully,  at  first  with  hot  dilute  hydrochloric  acid  and 
then  with  hot  water.  Reduce  with  stannous  chloride,  and 
titrate  with  standard  potassium  dichromate. 

Separate  Determination  of  Ferrous  and  Ferric  Iron 
in  a  Mineral. 

Ferrous  Iron. — The  exact  determination  of  ferrous  iron 
in  minerals  is  a  very  difficult  operation.  Reference  may  be 
made  to  Hillebrand's  Analysis  of  Silicate  and  Carbonate  Rocks, 
p.  154,  for  a  description  of  the  methods  employed  and  the 
difficulties  encountered. 

Certain  precautions  are  necessary  in  regard  to  the 
grinding,  as  it  is  found  that  considerable  oxidation  occurs  if 
the  sample  is  ground  to  a  fine  powder  in  the  ordinary 
manner.  A  very  fine  powder  is  usually  essential,  and 
oxidation  may  be  prevented  by  grinding  the  weighed  sample 
under  alcohol  in  an  agate  mortar.  The  alcohol  is  then 
allowed  to  evaporate  spontaneously. 

Solution  must  then  be  effected  in  absence  of  air.  If  the 
mineral  dissolves  in  hydrochloric  acid,  the  operation  may  be 
conducted  in  a  flask  which  has  been  filled  with  carbon 
dioxide  before  the  acid  is  added,  and  which  is  kept  filled  by 
passing  a  current  of  carbon  dioxide  into  the  flask  until 
solution  is  complete.  The  solution  is  then  cooled  in  an 
atmosphere  of  carbon  dioxide,  diluted  if  necessary,  and 
titrated. 

If  the  mineral  does  not  dissolve  readily  in  hydrochloric 
acid,  it  must  be  decomposed  in  absence  of  air  by  means  of 
hydrofluoric  acid  in  presence  of  sulphuric  acid.  (For  details, 
see  Hillebrand,  loc.  cit.) 

Ferric  Iron. — The  total  iron  is  then  determined  by  one  of 


CHROME  IRON  ORE  83 

the  methods  already  described,  and  the  difference  between 
the  total  iron  and  the  ferrous  iron  gives  the  amount  present 
in  the  ferric  state. 

Iron  in  Black  Ink. 

Black  inks  often  owe  their  colour  to  iron-tannin  com- 
pounds, and  the  organic  matter  must  be  destroyed  before  the 
iron  in  the  ink  can  be  determined. 

Weigh  (to  the  nearest  centigram)  10  grams  of  ink  in  a 
100  c.c.  porcelain  basin.  Evaporate  todryness  on  the  steam- 
bath,  and  then  heat  the  residue  and  burn  away  the  organic 
matter  at  as  low  a  temperature  as  possible.  Dissolve  the 
ash  by  warming  with  about  10  c.c.  of  concentrated  hydro- 
chloric acid,  adding,  if  necessary,  a  few  drops  of  stannous 
chloride  at  intervals.  Transfer  the  solution  to  a  300  c.c. 
beaker,  heat  to  the  boiling  point,  and  reduce  the  ferric  salt 
with  stannous  chloride.  Dilute  to  about  150  c.c.,  add  excess 
of  mercuric  chloride,  and  titrate  with  standard  dichromate. 

Express  the  result  in  grams  of  Fe2O3  per  100  grams 
of  ink. 

Iron  and  Chromium  in  Chrome  Iron  Ore. 

Decomposition  of  the  Ore. — Grind  the  ore  very  finely  in 
an  agate  mortar.  Weigh  accurately  about  0-5  gram,  transfer 
to  a  nickel  crucible,  and  mix  thoroughly  by  means  of  a  thin 
glass  rod  with  about  4  grams  of  sodium  peroxide.  Heat  the 
crucible  gently  with  a  very  small  Bunsen  flame  until  the 
contents  melt,  and  then  keep  the  mixture  at  low  redness  for 
about  ten  minutes.  Remove  the  flame  until  a  crust  forms, 
then  add  another  gram  of  sodium  peroxide,  and  heat  again 
to  low  redness  for  about  five  minutes. 

Place  the  crucible,  after  cooling,  in  a  porcelain  basin,  add 
about  50  c.c.  of  water,  and  warm  until  the  yellow  mass  has 
dissolved.  Remove  the  crucible  and  rinse  it.  If  the  solution 
is  purple  in  colour,  add  a  little  more  sodium  peroxide,  and 
then  boil  the  solution  in  the  covered  basin  until  the  excess 
of  sodium  peroxide  is  completely  decomposed.  In  order  to 
neutralise  the  large  excess  of  sodium  hydroxide  in  the 
solution,  add  about  5  grams  of  ammonium  carbonate  and 
boil  again.  Filter  and  wash  the  precipitate  thoroughly. 


84  VOLUMETRIC  ANALYSIS 

Determine  the  iron  in  the  precipitate  and  the  chromium  in 
the  filtrate. 

Iron. — Dissolve  the  precipitate  by  pouring  hot  hydro- 
chloric acid  (equal  volumes  of  concentrated  acid  and  water) 
into  the  filter,  and  receive  the  solution  in  a  300  c.c.  beaker. 
Wash  the  filter  with  hot  water.  (If  any  dark-coloured 
residue  remains,  it  must  be  fused  again  with  sodium  peroxide.) 
Reduce  the  ferric  salt  in  the  solution  with  stannous  chloride, 
and  titrate  with  standard  dichromate. 

Chromium. — Add  dilute  sulphuric  acid  to  the  filtrate 
until  it  is  acid  (orange-yellow),  and  then  add  25  c.c.  more 
acid.  Allow  the  solution  to  cool ;  then  reduce  the  dichromate 
by  adding  a  known  excess  of  ferrous  ammonium  sulphate,  as 
follows: — Place  several  grams  of  the  ferrous  salt  in  a  weigh- 
ing-bottle and  weigh  the  bottle  and  contents  ;  then  add  the 
salt  to  the  chromate  solution  gradually,  while  stirring,  until 
it  is  free  from  orange-yellow  colour,  and  until  a  small  drop  of 
the  solution  gives  a  blue  coloration  with  a  drop  of  freshly 
prepared  potassium  ferricyanide. 

Titrate  the  excess  of  ferrous  salt  in  the  solution  with 
standard  dichromate.  Also  weigh  the  weighing-bottle  again, 
and  thus  determine  the  total  weight  of  ferrous  ammonium 
sulphate  used. 


Standard   Iodine  and   Standard   Sodium 
Thiosulphate 

SODIUM  thiosulphate  is  used  mainly  for  the  determination 
of  iodine  or,  in  conjunction  with  a  standard  iodine  solution, 
for  the  determination  of  other  substances. 

The  interaction  between  iodine  and  sodium  thiosulphate 
is  as  follows  : — 

2Na2S2O3  +  2!  =  2NaI  +  Na2S4O6. 

The  process  may  be  used  for  the  determination  of  all 
substances  which  will  liberate  iodine  from  a  potassium  iodide 
solution,  and  is  therefore  capable  of  wide  application. 

DECINORMAL    SODIUM   THIOSULPHATE. 

(N/io  solution  contains  24.82  grams  Na^S^O^  $H<£)  per  litre.) 

The  so-called  "  pea-crystals  "  of  photographic  "  hypo  "  are 
usually  very  pure  Na2S2O3,  SH2O.  Many  specimens  are  so 
pure  that  an  accurately  decinormal  solution  may  be  prepared 
by  solution  of  24-82  grams  and  dilution  to  i  litre.  As, 
however,  the  salt  is  sometimes  impure  and  as  slight  decom- 
position occurs  if  the  water  contains  any  dissolved  carbon 
dioxide,  the  solution  should  be  standardised  a  few  days  after 
its  preparation.  After  any  dissolved  carbon  dioxide  has 
interacted  with  the  sodium  thiosulphate,  the  solution  is  quite 
stable  if  protected  from  atmospheric  carbon  dioxide. 

Titration  of  Iodine  with  Thiosulphate. — Run  in  the 
thiosulphate  from  a  burette.  When  the  iodine  solution 
becomes  very  pale  yellow  in  colour,  add  about  i  c.c.  of 
starch  solution,  and  continue  the  titration  until  the  blue 
colour  just  disappears.  The  starch  solution  must  not  be 
added  until  near  the  end  of  the  titration. 

85 


86  VOLUMETRIC  ANALYSIS 

Preparation  of  the  Starch  Solution. — Grind  about  I  gram 
of  starch  with  a  little  cold  water  until  it  forms  a  thin  paste. 
Pour  this,  drop  by  drop,  into  about  200  c.c.  of  boiling  water, 
and  boil  for  two  or  three  minutes.  Filter,  and  cool.  This 
starch  solution  will  not  keep  in  good  condition  for  more  than 
two  days,  but  will  keep  longer  if  the  solution  is  saturated 
with  sodium  chloride.  It  should  give  a  pure  blue  coloration 
with  a  trace  of  iodine ;  if  it  gives  a  purple  or  violet 
coloration,  the  starch  is  impure  and  is  unsuitable  for  the 
purpose.  "  Soluble  starch,"  which  may  be  dissolved  in  warm 
water  as  required,  is  very  satisfactory  if  a  pure  specimen 
is  obtainable. 

Standardisation  of  Sodium  Thiosulphate  with  pure 
Iodine. — Commercial  iodine  is  often  impure,  and  should  be 
ground  up  with  a  little  solid  potassium  iodide  and  resublimed 
before  use.1  On  account  of  its  volatility,  iodine  cannot  be 
weighed  in  the  ordinary  manner,  and  the  following  special 
method  must  therefore  be  adopted. 

In  a  weighing  -  bottle  place  about  2  grams  of  pure 
potassium  iodide  with  10  drops  of  water,  and  weigh 
accurately.  Add  0-3  to  0-4  gram  of  pure  iodine,  replace  the 
stopper  at  once,  and  weigh  again.  This  gives  the  weight  of 
the  iodine.  When  the  iodine  has  dissolved,  wash  it  rapidly 
into  a  flask  containing  about  i  gram  of  potassium  iodide  in 
about  200  c.c.  of  water,  and  titrate  at  once  with  the  sodium 
thiosulphate,  as  described  above. 

Repeat  the  process  with  another  weighed  quantity  of 
iodine,  and  from  the  results  calculate  the  concentration  of 
the  sodium  thiosulphate  solution. 

Standardisation  of  Sodium  Thiosulphate  with  Potassium 
Permanganate. — If  potassium  permanganate  is  added  to 
an  acid  solution  of  potassium  iodide,  the  permanganate 
is  reduced  and  an  equivalent  amount  of  iodine  is  liberated. 
A  solution  of  sodium  thiosulphate  can  therefore  be  standard- 
ised by  titrating  the  iodine  liberated  from  potassium  iodide 

1  A  small  quantity  of  pure  iodine  may  be  prepared  as  follows  :— 
Grind  together  in  a  mortar  about  2  grams  of  iodine  with  about  02  gram 
of  potassium  iodide.  Place  the  mixture  in  a  small  porcelain  basin,  cover 
with  a  clock-glass,  and, heat  gently.  Regulate  the  heat  so  that  the 
iodine  sublimes  slowly  on  to  the  clock-glass. 


STANDARD  SODIUM  THIOSULPHATE  87 

by  a  measured  volume  of  standard  potassium  permanganate 
solution. 


2KMnO4  +  ioKI  +  i6HCl  =  I2KC1  +  2MnCl2  +  10!  +  8H2O. 

Dissolve  about  2  grams  of  potassium  iodide  in  10  c.c.  of 
water,  and  add  10  c.c.  of  dilute  hydrochloric  acid.  To  this 
mixture  add  25  c.c.  of  standard  (decinormal)  potassium  per- 
manganate, dilute  to  about  200  c.c.,  and  titrate  the  liberated 
iodine  at  once  with  the  thiosulphate  solution. 

Standardisation  of  Sodium  Thiosulphate  -with  Potassium 
Bichromate.  —  When  potassium  dichromate  is  added  to  an 
acidified  solution  of  potassium  iodide,  iodine  is  liberated 
according  to  the  equation  : 

K2Cr2O7  +  6KI  +  i4HCl  =  8KC1  +  2CrCl3  +  61  +  yH.O. 

Each  equivalent  of  potassium  dichromate  liberates  an 
equivalent  of  iodine. 

If  a  standard  (decinormal)  solution  of  potassium  dichromate 
is  available,  it  may  be  used  for  the  standardisation  of  the 
thiosulphate  solution,  the  procedure  being  the  same  as  with 
standard  permanganate  (see  above).  Instead  of  a  standard 
solution,  weighed  quantities  of  potassium  dichromate  may 
be  used. 

Dissolve  about  2  grams  of  potassium  iodide  in  10  c.c.  of 
water  and  add  10  c.c.  of  dilute  hydrochloric  acid.  To  this  solu- 
tion add  012  to  0-14  gram  (accurately  weighed)  of  potassium 
dichromate,  dilute  to  about  200  c.c.,  and  titrate  the  liberated 
iodine  at  once  with  the  thiosulphate  solution.  At  the  con- 
clusion of  this  titration  the  solution  is  green  in  colour,  on 
account  of  the  chromic  chloride  present  The  colour  change 
at  the  end-point  —  from  blue  to  light  green—  is  nevertheless 
easily  observed  if  the  solution  is  diluted  to  at  least  200  c.c. 

The  gram-equivalent  of  potassium  dichromate  is  49-03. 

Standardisation  of  Sodium  Thiosulphate  with  Pure 
Copper.  —  This  is  described  on  p.  90. 


88  VOLUMETRIC  ANALYSIS 


DEOINORMAL    IODINE. 

solution  contains  12.692  grams  per  litrel) 

Iodine  is  almost  insoluble  in  water,  but  dissolves  in  a 
solution  of  potassium  iodide.  It  is  very  volatile,  and  both  in 
the  preparation  and  use  of  a  standard  iodine  solution,  pre- 
cautions are  necessary  to  prevent  loss  by  volatilisation. 
Commercial  iodine  is  usually  impure,  and,  even  if  pure  iodine 
is  available,  it  is  difficult  (on  account  of  its  volatility)  to 
prepare  an  accurate  standard  solution  by  weight.  It  is 
preferable,  therefore,  to  prepare  an  approximately  decinormal 
solution  from  commercial  (B.P.)  iodine,  and  standardise  it  by 
means  of  arsenious  oxide,  or  with  a  standard  sodium  thio- 
sulphate  solution. 

On  a  rough  balance  weigh  6-4  grams  of  powdered  iodine, 
and  introduce  it  into  a  500  c.c.  standard  flask.  Add  10  to  12 
grams  of  potassium  iodide  (free  from  iodate),  and  not  more 
than  20  c.c.  of  water.  Shake  until  all  the  iodine  has  dis- 
solved, and  then  dilute  to  the  graduation  mark.  Iodine 
dissolves  quickly  in  a  concentrated  potassium  iodide  solution, 
but  very  slowly  in  a  dilute  solution  (much  time  will  there- 
fore be  wasted  if  the  solution  is  diluted  before  all  the  iodine 
has  dissolved). 

Standardisation  -with  Arsenious  Oxide.  —  The  arsenious 
oxide  must  be  resublimed  before  use,  unless  of  known  purity. 
Weigh  accurately  about  1-237  gram  in  a  porcelain  basin,  and 
dissolve  in  a  little  warm  concentrated  sodium  hydroxide 
solution.  Pour  the  solution  and  washings  through  a  funnel 
into  a  250  c.c.  standard  flask.  Add  I  c.c.  of  phenolphthalein 
and  then  dilute  sulphuric  acid  until  the  pink  colour  dis- 
appears. Dissolve  about  5  grams  of  sodium  bicarbonate  in 
about  100  c.c.  of  water,  filter  if  necessary,  and  add  this  to  the 
solution  in  the  flask.  If  the  solution  is  pink  in  colour,  add 
dilute  sulphuric  acid,  drop  by  drop,  until  the  colour  is  dis- 
charged. Mix  the  solution  thoroughly,  and  dilute  to  the 
graduation  mark  with  water. 

The  arsenite  solution  is  placed  in  the  burette,  and  a 
measured  volume  of  the  iodine  solution  is  titrated  with  it 


STANDARD  IODINE  AND  THIOSULPHATE          89 

in  the  usual  manner  (see  p.  85).     The  reaction  with  arsenious 
acid  and  iodine  is 

4!  +  As,O3  +  2H2O  ^^  4HI  +  As,O5. 

The  reaction  is  quantitative  if  the  hydriodic  acid  is  neutral- 
ised as  it  is  formed.  Sodium  bicarbonate  will  effect  this, 
without  at  the  same  time  introducing  an  error  by  inter- 
action with  the  iodine ;  neither  sodium  hydroxide  nor  sodium 
carbonate  may  be  used  for  the  purpose,  as  they  interact  with 
iodine. 
,  The  gram-equivalent  of  arsenious  oxide  is  49-48. 

Note. — The  standard  sodium  arsenite  solution  may  also 
be  used  for  the  determination  of  the  available  chlorine  in 
bleaching  powder  (see  p.  107). 

ANALYSES    INVOLVING   THE    USE    OP    STANDARD 
IODINE   AND    STANDARD    THIOSULPHATE. 

Copper. 

When  a  copper  salt  is  mixed  with  a  solution  of  potas- 
sium iodide,  cuprous  iodide  is  precipitated  and  iodine  is 
liberated  : 

2CuSO4  +  4KI  =  2CuI  +  2K2SO4  +  2!. 

The  amount  of  copper  can  therefore  be  found  by  titration 
of  the  free  iodine.  According  to  the  equation,  I  gram  of 
copper  requires  5-22  grams  of  potassium  iodide,  but  in 
practice  a  considerable  excess  of  the  latter  is  necessary;  if 
the  solution  to  be  titrated  contains  about  0-15  gram  of 
copper — requiring  about  25  c.c.  of  standard  (decinormal) 
thiosulphate — about  2  grams  of  potassium  iodide  should  be 
added. 

Exercise. — The  copper  in  a  copper  or  "  nickel "  coin  may 
be  determined  by  this  method.  Clean  a  halfpenny  with 
emery  cloth,  cut  in  half,  and  weigh  one  portion  accurately. 
Place  it  in  a  200  c.c.  conical  flask,  dissolve  in  a  mixture  of 
concentrated  nitric  acid  and  water  in  equal  volumes,  and 
boil  to  expel  oxides  of  nitrogen.  The  small  white  residue  of 
stannic  oxide  need  not  be  filtered  off,  as  it  in  no  way  inter- 
feres with  the  analysis. 


90  VOLUMETRIC  ANALYSIS 

It  is  essential  that  the  solution  to  be  titrated  should 
contain  no  nitrite  and  no  free  acid  other  than  acetic  acid. 
Add  ammonia,  therefore,  in  slight  excess,  and  then  boil  until 
the  odour  of  ammonia  becomes  faint ;  then  add  more  than 
sufficient  acetic  acid  to  dissolve  the  precipitate,  and  boil 
again  for  two  minutes.  Cool  and  dilute  the  solution  to  500 
c.c.  in  a  standard  flask,  and  take  portions  of  25  c.c.  for  each 
titration. 

In  a  200  c.c.  conical  flask  dissolve  2  grams  of  potassium 
iodide  in  20  c.c.  of  water,  and  add  25  c.c.  of  the  copper  solu- 
tion. (Cuprous  iodide  is  nearly  white,  but  the  free  iodine 
colours  the  mixture  brown.)  Titrate  the  mixture  with 
standard  sodium  thiosulphate  until  the  brown  colour  becomes 
faint ;  then  add  i  c.c.  of  starch  solution,  and  continue  the 
titration  until  the  mixture  loses  the  last  trace  of  a  blue  tinge 
and  appears  almost  white.  The  end-point  is  quite  sharply 
defined.  After  reading  the  burette,  however,  it  will  be  found 
advantageous — until  some  experience  has  been  gained — to 
add  a  few  additional  drops  of  the  thiosulphate  and  to  keep 
the  mixture  as  a  guide  for  a  second  titration. 

The  reappearance  of  the  blue  colour  soon  after  the 
titration  is  apparently  finished  indicates  that  the  solution 
contains  nitrite,  or  that  insufficient  potassium  iodide  was 
added — in  short,  that  the  above  directions  have  not  been 
carefully  followed. 

Calculate  the  percentage  of  copper  in  the  coin.  One 
litre  of  normal  thiosulphate  corresponds  to  63-57  grams  of 
copper. 

Standardisation  of  Sodium  Thiosulphate  with  Pure 
Copper. — If  a  standard  solution  of  sodium  thiosulphate  solu- 
tion is  to  be  used  mainly  for  the  determination  of  copper, 
it  is  best  to  standardise  it  in  the  following  manner : — Weigh 
accurately  about  o  1 5  gram  of  pure  (electrolytic)  copper  foil. 
Place  it  in  a  200  c.c.  flask,  and  dissolve  in  about  3  c.c.  of 
concentrated  nitric  acid.  Dilute  the  solution  with  a  little 
water,  and  boil  to  expel  oxides  of  nitrogen.  Then — following 
the  method  detailed  in  the  preceding  paragraph — add 
ammonia,  and  boil ;  add  acetic  acid,  and  boil  again  ;  cool,  add 
2  grams  of  potassium  iodide,  and  titrate  the  mixture  with 
the  thiosulphate. 


STANDARD  IODINE  AND  THIOSULPHATE          91 

Sulphurous  Acid  and  Sulphites. 

Sulphurous  acid  in  dilute  aqueous  solution  is  oxidised  by 
iodine  to  sulphuric  acid  according  to  the  equation  : 


The  alkali  sulphites  are  oxidised  to  sulphates  in  a  similar 
manner. 

The  sulphite  solution  is  run  into  a  measured  excess 
of  iodine  (not  vice  versa),  with  constant  stirring,  and  the 
residual  iodine  is  then  titrated  with  standard  thiosulphate. 
If  the  iodine  is  run  into  the  sulphite  solution,  the  reaction 
takes  place  in  accordance  with  the  above  equation  only  when 
the  sulphite  solution  is  very  dilute  (about  centinormal). 

Exercise.  —  Determine  the  percentage  of  Na2SO3  in  a 
sample  of  commercial  sodium  sulphite  crystals.  Weigh 
accurately  about  3  grams  of  the  crystals,  dissolve  in  water, 
and  dilute  to  250  c.c.  in  a  standard  flask. 

Measure  25  c.c.  of  decinormal  iodine  into  a  flask,  add  5 
c.c.  of  dilute  hydrochloric  acid,  dilute  to  about  100  c.c.,  and 
add  slowly  25  c.c.  of  the  sulphite  solution.  Titrate  the  excess 
of  iodine  with  standard  thiosulphate. 

Hydrogen  Sulphide. 

Hydrogen  sulphide  interacts  with  iodine  in  aqueous  solu- 
tion according  to  the  equation  : 


A  measured  volume  of  the  hydrogen  sulphide  solution  is  run 
into  excess  of  decinormal  iodine,  and  the  excess  of  the  latter 
is  then  titrated  with  standard  thiosulphate. 

If  the  concentration  of  the  hydrogen  sulphide  solution  is 
more  than  about  0-025  normal,  the  precipitated  sulphur 
encloses  a  portion  of  the  iodine,  which  is  thereby  protected 
from  interaction  with  the  thiosulphate  ;  the  titration  is  then 
inaccurate.  Accordingly,  after  making  a  preliminary  titra- 
tion, the  hydrogen  sulphide  solution  must  be  diluted  in  a 
standard  flask  with  air-free  water  in  such  proportions  that  10 
c.c.  of  decinormal  iodine  will  oxidise  about  40  c.c.  of  the 
sulphide  solution. 


92  VOLUMETRIC  ANALYSIS 

In  order  to  determine  the  amount  of  hydrogen  sulphide 
in  mineral  waters,  take  a  measured  volume,  say  5  c.c.,  of 
decinormal  (or  centinormal)  iodine,  add  starch  solution  and 
2  grams  of  potassium  iodide,  and  pour  in  the  water  from  a 
measuring  cylinder  until  the  blue  colour  is  discharged. 
Titrate  back  with  decinormal  (or  centinormal)  iodine.  A 
correction  is  necessary  for  the  amount  of  iodine  required  to 
produce  a  blue  colour  in  absence  of  hydrogen  sulphide.  In 
order  to  determine  this,  add  starch  solution  and  2  grams  of 
potassium  iodide  to  a  quantity  of  distilled  water  equal  in 
volume  to  that  of  the  mineral  water  used,  and  titrate  with 
the  iodine. 

Exercise. — Determine  the  solubility  of  hydrogen  sulphide 
in  water  at  room  temperature. 

Peroxides,  Chromates,  Chlorates,  etc. 

Substances  which  oxidise  hydrochloric  acid  with  evolu- 
tion of  chlorine  may  be  accurately  determined  in  the  follow- 
ing manner.  The  method  is  specially  useful  for  peroxides, 
such  as  lead  peroxide,  red  lead,  and  manganese  dioxide,  and 
as  an  illustration  the  determination  of  manganese  dioxide 
in  pyrolusite  is  described. 

Valuation  of  Pyrolusite. — Manganese  dioxide  interacts 
with  hydrochloric  acid  according  to  the  equation : 

MnO2  +  4HC1  -  MnCl2  +  2H2O  +  C12. 

If  the  chlorine  is  passed  into  potassium  iodide  solution,  it 
liberates  an  equivalent  amount  of  iodine  which  may  be 
determined  by  titration  with  standard  sodium  thiosulphate 
solution. 

Reduce  some  pyrolusite  to  a  fine  powder  by  thorough 
grinding.  In  a  small  weighing-tube  (made  by  closing  one 
end  of  a  piece  of  glass  tube  f  inch  long)  weigh  accurately 
about  02  gram  of  the  powder,  and  introduce  the  tube  and 
contents  into  a  200  c.c.  distillation  flask  (Fig.  28). 

Fit  the  flask  with  a  cork  and  glass  tube,  so  arranged  that 
a  current  of  carbon  dioxide  from  a  Kipp  apparatus  can  be 
passed  into  the  flask  and  through  the  solution  that  is  to  be 
boiled  in  it.  Connect  the  bent  side-tube  of  the  flask  with  a 
U-tube  which  contains  2  grams  of  potassium  iodide  dissolved 


STANDARD  IODINE  AND  THIOSULPHATE 


93 


in  just  sufficient  water  to  fill  the  bend  of  the  tube.  As  a 
precaution  against  incomplete  absorption,  connect  with  the 
U-tube  a  tube  full  of  glass  beads  wetted  with  potassium 
iodide  solution.  Place  the  U-tube  in  a  basin  of  cold 
water. 

When  the  apparatus  is  ready,  pour  10  c.c.  of  water  and 
20  c.c.  of  concentrated  hydrochloric  acid  into  the  flask,  and 
replace  the  cork  at  once.  Heat  the  mixture  very  gently  so 
that  chlorine  is  slowly  evolved.  Gradually  increase  the 


FIG.  28. 


temperature,  and  finally  heat  until  boiling,  and  pass  a  slow 
current  of  carbon  dioxide  through  the  boiling  solution  until 
all  the  chlorine  is  driven  out  of  the  flask  (about  ten  minutes 
as  a  rule).  In  order  to  prevent  the  iodine  and  potassium 
iodide  solution  from  passing  back  into  the  flask,  increase  the 
current  of  carbon  dioxide  immediately  the  heating  is 
stopped.  As  pyrolusite  invariably  contains  iron,  the  solution 
in  the  flask  remains  yellow  at  the  end  of  the  operation. 

Disconnect  the  absorption  tube,  wash  the  iodine  and 
potassium  iodide  solution  into  a  beaker,  and  titrate  at  once 
in  the  usual  manner  with  sodium  thiosulphate. 

Calculate  the  percentage  of  MnO2  in  the  sample. 


94  VOLUMETRIC  ANALYSIS 

Available  Chlorine  in  Bleaching  Powder. 

Bleaching  powder  may  be  regarded  as  a  mixed  salt, 
Ca(OCl)Cl,  which,  in  solution  and  so  far  as  its  behaviour  in 
analytical  work  is  concerned,  is  similar  to  an  equimolecular 
mixture  of  calcium  chloride  and  calcium  hypochlorite, 
CaCl2  •  Ca(OCl)2.  When  treated  with  acid,  the  whole  of  the 
chlorine  in  this  mixed  salt,  amounting  to  about  43  per  cent, 
is  liberated,  and  is  therefore  "available."  The  best  com- 
mercial samples,  however,  seldom  contain  more  than  36  to 
38  per  cent,  of  available  chlorine.  Bleaching  powder  also 
decomposes  slowly  on  -keeping,  with  formation  of  calcium 
chloride  and  chlorate, 

2Ca(OCl)Cl  =  2CaCl2  +  0, 
6Ca(OCl)  Cl  -  sCaCl.2  +  Ca(ClO8)2 

whilst  exposure  to  atmospheric  moisture  and  carbon  dioxide 
results  in  loss  of  chlorine  and  hypochlorous  acid, 

Ca  (OC1)  Cl  +  H2CO3  -  CaCO3  +  H2O  +  C12 
2Ca(OCl)  Cl  +  H2CO8  -  CaCO3  +  CaCl2  +  2HOC1. 

The  available  chlorine  thus  diminishes  on  keeping,  and  is  no 
longer  equal  to  the  total  chlorine  (see  p.  105). 

The  amount  of  available  chlorine  may  be  determined  by 
mixing  a  solution  of  the  bleaching  powder  with  excess  of 
potassium  iodide  and  adding  acid.  The  reaction  is  : 

/Ca  (OC1)  Cl  +  H2S04  =  CaSO4  +  H2O  +  Cl, 

\  Cl 


The  liberated  iodine,  which  is  equivalent  to  the  available 
chlorine,  is  then  titrated  with  standard  thiosulphate.  The 
procedure  is  as  follows  :  — 

Weigh  exactly  (in  a  stoppered  weighing-bottle)  about 
5  grams  of  bleaching  powder.  Bleaching  powder  is  not 
completely  soluble  in  water;  in  order  to  obtain  a  uniform 
sample,  it  must  be  so  finely  ground  that  the  insoluble  portion 
will  remain  for  some  time  in  suspension.  Transfer  the 
weighed  sample  to  a  glazed  porcelain  mortar,  add  2  or  3  c.c. 
of  water,  and  grind  to  a  smooth  paste.  Add  more  water 
gradually,  then  transfer  the  mixture  completely  to  a  500  c.c. 
standard  flask  and  make  up  to  the  mark.  Mix  the  contents 


TIN  IN  AN  ALLOY  95 

of  the  flask  by  shaking,  and  repeat  the  shaking  immediately 
before  withdrawing  each  sample  for  titration. 

Measure  25  c.c.  of  the  mixture  into  a  200  c.c.  flask,  add 
about  i  gram  of  potassium  iodide  (10  c.c.  of  a  10  per  cent, 
solution),  and  excess  of  acetic  acid.  Titrate  the  iodine  with 
standard  thiosulphate. 

Alternative  method. — The  available  chlorine  in  bleaching 
powder  may  also  be  determined  by  means  of  standard 
sodium  arsenite  (see  p.  107). 

Tin  in  an  Alloy. 

Preparation  of  a  Solution  for  Analysis. — (i)  If  the  alloy 
is  soluble  in  hydrochloric  acid,  dissolve  a  weighed  portion 
(from  01  gram  upwards,  according  to  the  amount  of  tin) 
in  concentrated  hydrochloric  acid. 

(2)  If  the  alloy  is  not  completely  soluble  in  hydrochloric 
acid,  dissolve   it   in   a   mixture   of   10   c.c.   of  concentrated 
sulphuric  acid,  10  c.c.  of  concentrated   nitric  acid,  and   30 
c.c.  of  water.     Evaporate  the  solution  in  a  porcelain  basin 
or  casserole  until  dense  white   fumes  of  sulphuric  acid  are 
evolved,  cool,  and  transfer  the  solution  to  a  400  c.c.  conical 
flask,  using  cold  water  to  wash  the  basin. 

(3)  If  the  alloy  cannot  be  brought  into  solution  by  either 
of  the  above  methods,  disintegrate  it   with   nitric  acid   as 
described  on  p.  223,  dilute  to  about  30  c.c.,  filter,  and  wash 
the  residue  with  hot  water.     Analyse  the  residue  according 
to  the  method  given  below  for  an  ore  (p.  96). 

Reduction  and  Titration. — The  tin  is  reduced  to  the 
stannous  condition  by  boiling  with  metallic  antimony  and 
hydrochloric  acid.  The  stannous  salt  is  then  determined 
by  titration  with  standard  iodine  solution,  which  oxidises 
it  to  the  stannic  condition. 

Stannous  chloride  is  very  readily  oxidised  by  atmospheric 
oxygen,  and  it  is  essential,  therefore,  that  the  solution  should 
be  protected  from  contact  with  the  air  both  before  and 
during  the  titration.  This  is  accomplished  by  keeping  the 
flask  filled  with  carbon  dioxide. 

Both  arsenic  and  antimony  in  the  trivalent  state  can  be 


96  VOLUMETRIC  ANALYSIS 

oxidised  by  iodine  under  certain  conditions,  but  they  do  not 
interact  with  iodine  in  strongly  acid  solution,  and  therefore 
do  not  interfere  with  the  determination  of  tin  by  this  method. 

Procedure. — Place  the  solution  in  a  400  c.c.  conical  flask 
fitted  with  a  rubber  cork  carrying  inlet  and  outlet  tubes, 
whereby  the  flask  can  be  completely  filled  with  carbon 
dioxide. 

To  the  tin  solution  in  the  conical  flask,  add  50  c.c.  of 
concentrated  hydrochloric  acid,  and  dilute  to  about  200  c.c. 
with  hot  water.  Add  about  I  gram  of  antimony  powder,  fill 
the  flask  with  carbon  dioxide,  and  boil  the  solution  gently 
for  thirty  minutes.  Cool  the  solution  and  pass  the  carbon 
dioxide  briskly  through  the  flask  during  the  cooling  process 
to  prevent  access  of  air.  When  cold,  add  some  starch 
indicator  solution  and,  without  filtration  from  the  excess  of 
antimony,  titrate  the  solution  with  decinormal  iodine.  The 
carbon  dioxide  should  be  passed  into  the  flask  throughout 
the  titration  process,  the  rubber  cork  being  lifted  only  so 
far  as  to  permit  the  introduction  of  the  tip  of  the  burette. 

The  end-point  in  the  titration  is  attained  when  a  blue 
coloration  throughout  the  solution  persists  for  a  few  seconds. 
The  colour  is  subsequently  discharged  on  account  of  inter- 
action between  the  iodine  and  antimony ;  this  reaction, 
however,  is  comparatively  slow,  and  does  not  affect  the 
accuracy  of  the  titration. 

Tin  in  an  Ore. 

Preparation  of  a  Solution  for  Analysis. — Fuse  about  8 
grams  of  potassium  hydroxide  in  a  spun  iron  crucible,  and 
continue  the  heating  until  all  moisture  is  expelled  and  quiet 
fusion  attained.  Cool,  add  about  0-5  gram  (exactly  weighed) 
of  the  finely  powdered  ore,  cover  the  crucible,  and  heat,  at 
first  cautiously,  and  then  with  a  full  Bunsen  flame  until  action 
ceases.  Pour  the  molten  mass  into  a  clean  nickel  basin 
floating  in  a  dish%of  water,,and  cover  the  hot  mass  with  a 
crucible  lid  to  prevent  loss  when  the  mass  cracks  during 
cooling. 

Place  the  iron  crucible  in  a  porcelain  basin,  add  100  c.c. 
of  water,  and  boil.  If  any  of  the  fused  mass  remains  attached 


TIN  IN  AN  OftE  9? 

to  the  crucible,  add  a  little  hydrochloric  acid,  and  assist  the 
solution  process  by  breaking  off  any  adhering  lumps  with 
a  glass  rod.  When  the  crucible  is  clean,  remove  and  wash 
it.  To  the  solution  add  the  detached  cake,  together  with 
the  crucible  lid  if  there  is  anything  attached  to  it.  Add 
30  c.c.  of  concentrated  hydrochloric  acid  and  boil.  There 
should  be  no  undissolved  residue  from  the  ore,  but  there 
may  be  a  few  scales  of  ferric  oxide  from  the  crucible. 

Transfer  to  a  400  c.c.  conical  flask,  add  30  c.c.  of  con- 
centrated hydrochloric  acid,  dilute  to  about  200  c.c.  with  hot 
water,  and  proceed  at  once  with  the  reduction  and  titration 
as  described  on  p.  95. 


G 


Standard  Silver  Nitrate  and  Potassium 
Thiocyanate 

DECINORMAL  SILVER  NITRATE. 

(Nj  10  solution  contains  16.99  grams  AgNO^per  litre.} 


STANDARD  silver  nitrate  solution  is  used  mainly  for  the  deter- 
mination of  chloride  —  (i)  by  direct  titration,  with  potassium 
chromate  as  indicator  ;  or  (2)  in  conjunction  with  standard 
potassium  (or  ammonium)  thiocyanate  solution,  with  a  ferric 
salt  as  indicator.  The  first  method  can  be  used  only  if  the 
chloride  solution  is  neutral.  The  same  standard  solutions 
can  also  be  used  for  the  determination  of  bromide,  iodide, 
and  cyanide  ;  chlorate,  bromate,  and  iodate  ;  silver,  and 
mercury. 

A  solution  made  by  dissolving  16-99  grams  of  pure  silver 
nitrate  in  water  and  diluting  to  I  litre,  is  accurately  deci- 
normal.  If,  however,  ordinary  commercial  silver  nitrate  is 
used,  the  solution  should  be  standardised  with  potassium  or 
sodium  chloride  of  known  purity. 

Standardisation  of  Silver  Nitrate  Solution.  —  If  the  silver 
nitrate  is  to  be  used  in  conjunction  with  standard  potassium 
thiocyanate,  it  must  be  standardised  by  the  method  given  on 
p.  102  ;  otherwise,  it  may  be  standardised  with  pure  potassium 
(or  sodium)  chloride,  using  potassium  chromate  as  indicator. 

Dry  the  potassium  chloride  by  heating  it  gently  in  a 
porcelain  basin,  or  fuse  it  in  a  platinum  basin  and  break  up 
the  fused  mass.  Weigh  accurately  about  1-8  gram,  dissolve 
in  water,  and  make  up  to  250  c.c.  in  a  standard  flask. 
Titrate  25  c.c.  with  the  silver  nitrate  solution,  as  described 
in  the  next  paragraph. 

Titration  of   a    Chloride   in   Neutral   Solution.  —  Dilute 


STANDARD  SILVER  NITRATE  99 

the  chloride  solution  to  about  70  c.c.  in  a  porcelain  basin, 
and  add  I  c.c.  of  a  2  per  cent,  solution  of  neutral  potassium 
chromate1  (free  from  chloride).  Run  the  silver  nitrate  slowly 
into  the  chloride  solution,  with  constant  stirring.  Silver 
chloride  is  precipitated,  and  a  red  precipitate  of  silver 
chromate  also  forms  locally,  but  disappears  on  stirring. 
After  all  the  chloride  is  precipitated,  however,  the  addition 
of  more  silver  nitrate  produces  a  permanent  precipitate  of 
silver  chromate.  Continue  the  titration,  therefore,  until  a 
faint  reddish  tinge  persists  even  after  brisk  stirring.  This 
method  is  accurate  only  with  cold  and  neutral  or  very 
slightly  alkaline  solutions.  If  the  solution  is  acid,  it  is 
usually  permissible  to  neutralise  it  by  adding  a  slight  excess 
of  pure  calcium  carbonate. 

ANALYSES  INVOLVING  THE  USE  OP  STANDARD 
SILVER  NITRATE. 

Chloride  and  Bromide. 

The  chlorides  (and  bromides)  of  sodium,  potassium, 
ammonium,  magnesium,  and  calcium,  in  neutral  solution,  may 
be  determined  by  titration  with  standard  silver  nitrate,  using 
potassium  chromate  as  indicator.  The  procedure  is  described 
above.  For  iodide,  the  method  is  less  satisfactory. 

Chloride  in  Barium  Chloride. 

Some  modification  of  the  ordinary  procedure  is  necessary 
in  this  case,  since  barium  chromate  is  insoluble. 

Add  to  the  barium  chloride  solution  sufficient  pure 
potassium  sulphate  to  precipitate  all  the  barium  as  barium 
sulphate.  Dilute  to  about  70  c.c.,  add  i  c.c.  of  the  chromate 
indicator,  and  titrate  in  the  usual  manner  without  filtering  off 
the  barium  sulphate. 

Cyanide. 

When  silver  nitrate  is  added  to  a  solution  of  an  alkali 
cyanide,  it  produces  a  local  precipitate  of  silver  cyanide 

1  Dilute  20  c.c.  of  the  bench  solution  to  50  c.c.  If  the  solution  con- 
tains chloride,  add  a  few  drops  of  silver  nitrate,  and  filter. 


100  VOLUMETRIC  ANALYSIS 

which,  however,  dissolves  in  the  excess  of  the  alkali  cyanide, 
and  forms  a  soluble  complex  cyanide  : 

f  KCN  +  AgNO3  =  AgCN  +  KNO3 
I  AgCN  +  KCN  =  KAg(CN).2. 

After  all  the  cyanide  has  been  converted  into  this  complex 
salt,  the  addition  of  more  silver  nitrate  produces  a  permanent 
precipitate  of  silver  cyanide  : 


2. 


The  cyanide  in  a  solution  may  therefore  be  determined 
by  titration  with  standard  silver  nitrate  until  a  permanent 
precipitate  is  produced.  This  indicates  that  all  the  cyanide 
has  been  converted  into  the  complex  salt,  and  marks  the 
beginning  of  reaction  2.  The  addition  of  a  few  drops  of 
potassium  iodide  makes  the  end-point  sharper.  In  presence 
of  ammonium  salts,  silver  cyanide  is  not  precipitated,  and 
potassium  iodide  must  be  added  as  an  indicator. 

Procedure.  —  Add  excess  of  sodium  hydroxide  and  a  few 
drops  of  potassium  iodide  to  the  cyanide  solution  contained 
in  a  beaker,  and  dilute  the  mixture  until  the  concen- 
tration of  the  cyanide  solution  is  less  than  0-02  normal. 
Place  the  beaker  on  a  sheet  of  black  paper,  and  run  the 
standard  silver  nitrate  slowly  into  the  cyanide  solution, 
with  constant  stirring,  until  the  first  permanent  opalescence 
appears. 

One  molecule  of  silver  nitrate  corresponds  to  two 
molecules  of  potassium  cyanide,  i.e.,  i  c.c.  of  normal  silver 
nitrate  corresponds  to  0-1302  gram  of  potassium  cyanide. 

Determination  of  Hydrocyanic  Acid.  —  On  account  of  its 
volatility  and  the  poisonous  nature  of  the  vapour,  it  is 
advisable  to  titrate  weighed  portions  of  the  solution,  in  order 
to  avoid  the  use  of  a  pipette.  If  a  pipette  is  to  be  used, 
it  should  be  filled  by  mechanical  suction.  Excess  of  sodium 
hydroxide  must  be  added  to  the  acid  before  commencing 
the  titration. 

Determination  of  Cyanide  in  Commercial  Cyanide.  — 
Weigh  accurately  3  to  3-5  grams,  dissolve  in  water,  and 
make  up  to  250  c.c.  in  a  standard  flask.  Use  25  c.c.  of  the 
solution  for  each  titration. 


STANDARD  POTASSIUM,  TH-IQCYANATfii      : 

The  commercial  practice  is  to  express  the  result  as  so 
much  per  cent,  of  potassium  cyanide.  As  commercial 
cyanides  are  often  mainly  sodium  cyanide,  many  samples 
give  a  "percentage"  considerably  above  100. 


DECINORMAL  SILVER  NITRATE  AND  DECINORMAL 
POTASSIUM  THIOCYANATE. 

When  potassium  thiocyanate  is  added  to  silver  nitrate, 
a  white  precipitate  of  silver  thiocyanate  is  produced.  In 
order  to  mark  the  end-point,  a  ferric  salt  (free  from  chloride) 
is  added  to  the  silver  solution.  No  ferric  thiocyanate  is 
permanently  formed  until  all  the  silver  is  precipitated  as 
thiocyanate,  and  the  appearance  of  a  permanent  brownish 
coloration  (ferric  thiocyanate)  shows  when  precipitation  is 
complete.  The  titration  must  be  performed  in  acid  solution, 
and  nitric  acid  is  therefore  added  to  the  silver  solution. 

The  thiocyanates  of  the  alkalis  are  deliquescent  salts, 
and  a  standard  solution  cannot  be  prepared  by  weighing 
a  definite  quantity.  An  approximately  decinormal  solution 
is  therefore  made  by  dissolving  about  10  grams  of  potassium 
thiocyanate  (or  8  grams  of  ammonium  thiocyanate)  in  a 
litre,  and  this  solution  is  standardised  with  decinormal  silver 
nitrate. 

An  approximately  decinormal  solution  of  silver  nitrate 
is  required,  and  is  made  by  dissolving  17  grams  in  i  litre 
(see  p.  98). 

Preparation  of  the  Indicator  Solution. — Dissolve  5  grams 
of  iron  alum  in  50  c.c.  of  water,  add  50  c.c.  of  concentrated 
nitric  acid  (free  from  chloride),  and  boil  the  solution  vigorously 
to  expel  oxides  of  nitrogen.  Use  5  c.c.  of  the  solution  for 
each  titration. 

Titration  of  Silver  Nitrate  with  Potassium  Thio- 
cyanate.— The  thiocyanate  solution  must  be  run  into  the 
silver  nitrate  solution,  not  vice  versa.  Add  5  c.c.  of  the 
indicator  solution  to  25  c.c.  of  the  silver  nitrate  contained  in 
a  porcelain  basin,  dilute  to  about  75  c.c.,  and  then  run  in  the 
thiocyanate  slowly,  with  constant  stirring.  The  first  tinge  of 
a  permanent  brown  coloration  marks  the  end-point.  The 
colour  is  more  easily  seen  if  the  precipitate  is  allowed  to 


102  etfMtfTRIC  ANALYSIS 

settle.  Until  some  experience  has  been  gained,  it  is 
advisable,  after  completing  the  first  titration,  to  destroy  the 
brown  coloration  by  the  addition  of  about  I  c.c.  of  silver 
nitrate,  and  to  keep  this  mixture  as  a  guide  for  a  second 
titration  made  in  another  similar  basin.  The  first  appearance 
of  a  permanent  brown  coloration  is  more  easily  observed  by 
comparison  with  this  mixture  (in  which  the  end-point  has  not 
been  reached). 

After  the  silver  nitrate  solution  has  been  standardised 
(as  described  in  the  next  paragraph),  the  normality  of  the 
thiocyanate  solution  may  be  calculated. 

Standardisation  of  the  Silver  Nitrate. — Weigh  exactly 
0-17  to  o  1 8  gram  of  pure  dry  potassium  chloride,  or  0-13  to 
0-14  gram  of  sodium  chloride,  and  dissolve  in  20  to  30  c.c.  of 
water.  Add  about  i  c.c.  of  dilute  nitric  acid  and  25  c.c.  of 
the  silver  nitrate  solution.  Shake  or  stir  until  the  precipitate 
coagulates  and  leaves  the  supernatant  liquid  clear.  Filter, 
and  wash  the  precipitate  with  cold  water  until  all  the  silver 
nitrate  is  removed.  To  the  filtrate  and  washings  add  the 
ferric  indicator,  and  titrate  the  unused  silver  nitrate  with 
thiocyanate.  From  the  result,  calculate  the  volume  of  silver 
nitrate  solution  required  for  the  weighed  quantity  of  potassium 
chloride  taken,  and  from  this  the  normality  of  the  silver 
nitrate. 

Calculation. — It  was  found  that  24-10  c.c.  of  a  thiocyanate 
solution  was  required  for  25  c.c.  of  a  silver  nitrate  solution. 

Of  this  silver  nitrate,  25  cc.  was  added  to  0-1720  gram 
of  potassium  chloride,  and  the  filtrate  required  1-21  c.c.  of 
the  thiocyanate. 

The  excess  of  silver  nitrate  corresponds  to    I -21  c.c.  of 

the  thiocyanate  =  i-2i  x --=  1-25    c.c.  of  the   silver  nitrate 

solution. 

The  volume  of  the  silver  nitrate  solution  corresponding  to 
0-1720  gram  of  potassium  chloride  is,  therefore,  25  —  1-25 
=  23-75  c.c. 

—,       .,          ...  ,       e        1000x0-1720 

The  silver  nitrate  is,  therefore, —^  normal, 

23-75x74-6 

=  0-0971  N. 


CHLORIDE,  BROMIDE,  AND  IODIDE  103 


ANALYSES     INVOLVING     THE     USE     OP     STANDARD 
SILVER  NITRATE  AND  STANDARD  THIOCYANATE. 

Chloride,  Bromide,  and  Iodide. 

Chloride. — The  determination  of  a  chloride  is  carried  out 
in  a  manner  exactly  similar  to  the  method  of  standardisation 
described  above. 

To  the  chloride  solution,  acidified  with  nitric  acid,  add  a 
measured  volume  of  standard  silver  nitrate  in  slight  excess, 
and  stir  the  mixture  until  the  silver  chloride  coagulates  and 
settles.  Filter  the  silver  chloride,  wash  it  with  cold  water, 
and  titrate  the  excess  of  silver  nitrate  in  the  filtrate  with 
thiocyanate. 

The  following  points  must  be  attended  to  if  accurate 
results  are  to  be  obtained  : — 

1.  The  chloride  solution,  if  not   already  acid,  must   be 
acidified  with  nitric  acid  which  is  free  from   chloride  and 
nitrite. 

2.  The  silver  chloride  must  be  filtered  off  before  titrating 
the  excess  of  silver  nitrate.     The  reason  for  filtration  lies  in 
the  fact  that  silver  chloride  interacts  with  ferric  thiocyanate, 
with  formation  of  silver  thiocyanate  and  ferric  chloride. 

3.  By   using   as   small   an   excess   as   possible   of  silver 
nitrate,  any  error  due  to  incorrect   standardisation  of  the 
thiocyanate  is  minimised,  and  the  precipitated  silver  chloride 
is  more  easily  washed  free  from  silver  nitrate. 

Bromide. — In  this  case  it  is  unnecessary  to  filter  off  the 
silver  bromide  before  titrating  the  excess  of  silver  nitrate, 
since  the  interaction  of  silver  bromide  with  ferric  thiocyanate 
is  negligible.  Otherwise  the  procedure  is  the  same  as  for 
chloride. 

Iodide. — When  silver  nitrate  is  added  to  an  iodide 
solution,  the  precipitated  silver  iodide  encloses  a  considerable 
amount  of  the  soluble  iodide  or  of  the  silver  nitrate,  and  an 
error  in  the  titration  results.  The  procedure  must  therefore 
be  modified  as  follows  : — 

To  a  measured  volume  of  the  iodide  solution,  contained 
in  a  stoppered  bottle,  add  a  little  nitric  acid,  and  dilute  to 
about  250  c.c.  Add  standard  silver  nitrate  gradually  from 


104  VOLUMETRIC  ANALYSIS 

a  burette,  I  to  2  c.c.  at  a  time,  and  after  each  addition  insert 
the  stopper  and  shake  the  bottle  vigorously.  Continue  the 
addition  of  silver  nitrate  until  a  slight  excess  is  present, 
when  the  precipitate  coagulates  and  the  supernatant  liquid 
becomes  clear.  Then  add  5  c.c.  of  the  ferric  indicator,  and, 
without  filtering,  titrate  the  excess  of  silver  nitrate  with 
standard  thiocyanate. 

Chlorate. 

The  chlorate  is  reduced  to  chloride  by  gently  boiling 
with  a  considerable  excess  of  sulphurous  acid  solution,  or  by 
passing  a  current  of  sulphur  dioxide  through  the  hot 
solution  for  about  five  minutes.  The  excess  of  sulphur 
dioxide  is  then  expelled  by  passing  a  current  of  carbon 
dioxide  through  the  boiling  solution  for  about  twenty 
minutes.  The  chloride  in  the  solution  is  then  determined  in 
the  usual  manner  with  standard  silver  nitrate  and  thio- 
cyanate. 

If  there  is  any  chloride  in  the  chlorate,  a  separate  portion 
of  the  solution  must  be  titrated  without  previous  reduction, 
and  the  amount  of  chloride  so  found  must  be  deducted  from 
the  titre  representing  the  total  chlorate  plus  chloride. 

Silver. 

Silver  may  be  determined  by  titration  of  a  solution  with 
standard  thiocyanate  in  the  usual  manner.  With  the  excep- 
tion of  mercury,  the  presence  of  other  metals  does  not, 
as  a  rule,  interfere  with  the  titration. 

For  practice,  determine  the  percentage  of  silver  in  a 
silver  coin.  Dissolve  a  threepenny-piece  in  about  20  c.c. 
of  concentrated  nitric  acid,  dilute  with  an  equal  volume  of 
water,  and  boil  until  oxides  of  nitrogen  are  expelled. 
Dilute  the  solution  to  100  c.c.  in  a  standard  flask,  and  use 
25  c.c.  for  each  titration. 

Mercury. 

Mercury  may  be  determined  in  the  same  manner  as 
silver  by  titration  with  standard  thiocyanate.  The  solution 
must  contain  a  large  excess  of  nitric  acid,  and  the  mercury 
must  be  present  as  mercuric  nitrate.  The  original  substance 


CHLORINE  IN  BLEACHING  POWDER  105 

should  therefore  be  dissolved  in  hot  concentrated  nitric 
acid,  and,  after  making  up  to  a  definite  volume,  a  portion 
of  the  solution  should  be  tested  for  mercurous  nitrate  by 
adding  hydrochloric  acid. 

The  reaction  which  occurs  in  the  titration  is  as  follows : — 

Hg(NO3)2  +  2KCNS  =  Hg(CNS)2  +  2KNO3. 

As  mercuric  thiocyanate  is  somewhat  soluble  in  water,  it 
may  not  be  precipitated  ;  but  this  does  not  interfere  with  the 
titration,  and  no  ferric  thiocyanate  is  permanently  formed 
until  the  above  reaction  is  complete. 

Exercise.  —  Determine  the  percentage  of  mercury  in 
mercuric  oxide.  Weigh  accurately  about  I  gram  of 
mercuric  oxide.  Dissolve  in  concentrated  nitric  acid  and 
dilute  the  solution  to  100  c.c.  in  a  standard  flask.  To  25 
c.c.  of  the  solution  add  10  c.c.  of  concentrated  nitric  acid 
and  5  c.c.  of  the  ferric  indicator,  dilute  to  about  70  c.c., 
and  titrate  with  standard  thiocyanate; 

Total  Chlorine  in  Bleaching  Powder. 

Besides  "  available "  chlorine,  bleaching  powder  may 
contain  chlorine  as  chloride  and  chlorate  which  is  useless 
for  bleaching  purposes  (see  p.  94).  In  order  to  determine 
the  total  chlorine,  prepare  a  solution  of  the  bleaching  powder 
in  the  manner  described  on  p.  94,  and  proceed  as  follows : — 

Hypochlorite  and  Chloride.  —  Measure  25  c.c.  of  the 
solution  into  a  beaker,  add  10  c.c.  of  dilute  ammonia,  cover 
the  beaker,  and  boil  gently  for  about  five  minutes.  The 
hypochlorite  is  thus  converted  into  chloride,  and  nitrogen 
is  liberated : 

3Ca(OCl)2  +  4NH3  =  3CaCl2  +  6H2O  +  2N2. 

Add  excess  of  dilute  nitric  acid,  carefully  avoiding  loss 
through  effervescence,  and  boil  until  free  from  carbon 
dioxide.  Add  excess  of  decinormal  silver  nitrate  (25  c.c.), 
stir  until  the  precipitate  coagulates,  filter,  and  wash.  Titrate 
the  excess  of  silver  nitrate  in  the  filtrate  with  standard 
thiocyanate  in  the  usual  way. 

Hypochlorite,  Chloride,  and  Chlorate. — Boil  25  c.c.  of 
the  solution  with  dilute  ammonia  as  before,  and  pass  a 


106  VOLUMETRIC  ANALYSIS 

current  of  sulphur  dioxide  through  the  boiling  solution  for 
a  few  minutes,  in  order  to  reduce  the  chlorate  to  chloride. 
Acidify  with  dilute  sulphuric  acid  and  oxidise  the  excess  of 
sulphur  dioxide  by  carefully  adding  potassium  permanganate 
until  the  solution  is  faintly  pink.  All  the  chlorine  in  the 
bleaching  powder  is  now  present  in  the  solution  as  chloride, 
which  may  be  determined  by  means  of  standard  silver 
nitrate  and  thiocyanate  in  the  usual  manner. 

From  the  results  the  percentage  of  total  chlorine  in  the 
bleaching  powder  may  be  calculated,  and  also  (by  difference) 
the  percentage  of  chlorine  present  as  chlorate.  If  the 
available  chlorine  in  the  same  sample  has  been  determined 
(pp.  94  and  107),  the  amount  of  chlorine  (not  available)  present 
as  chloride  may  also  be  calculated. 


Various  Volumetric  Processes 

Available  Chlorine  in  Bleaching  Powder  by  means  of 
Standard  Sodium  Arsenite. 

THIS  method  depends  on  the  oxidation  of  sodium  arsenite 
to  sodium  arsenate  by  the  hypochlorite.  The  reaction  may 
be  represented  by  the  following  (simplified)  equation  : — 

2  Ca  (OC1)  Cl  +  As,O3  =  2  CaCl,  +  As2O5. 

Standard  sodium  arsenite  solution  (p.  88)  is  run  into  the 
bleaching  powder  solution  from  a  burette,  and  the  end  of  the 
reaction  is  determined  by  means  of  potassium  iodide-starch 
paper  ;  so  long  as  any  hypochlorite  remains  undecomposed, 
a  blue  stain  is  produced  on  the  test-paper  when  a  drop  of  the 
solution  is  brought  into  contact  with  it.  The  procedure  is  as 
follows : — 

Prepare  a  solution  of  the  bleaching  powder  in  the  manner 
described  on  p.  94.  After  shaking,  measure  25  c.c.  of  the 
turbid  mixture  into  a  small  beaker  and  make  a  rough 
titration  by  running  in  the  standard  sodium  arsenite,  rapidly 
at  first,  and  then  0-5  c.c.  at  a  time,  stirring  constantly,  until 
a  drop  of  the  solution  gives  no  blue  stain  when  placed 
on  potassium  iodide-starch  paper. 

Repeat  the  titration,  only  on  this  occasion  remove  no  test 
drops  until  within  about  i  c.c.  of  the  previously  determined 
end-point.  As  the  end-point  is  approached,  the  blue  stain 
becomes  less  pronounced,  and  is  last  seen  near  the  centre  of 
the  wet  spot  on  the  test-paper.  The  end-point  is  quite 
sharp. 

Calculate  the  percentage  of  available  chlorine  in  the 
bleaching  powder. 

Preparation  of  Potassium  Iodide- Starch  Paper. — Grind 
0-5  gram  starch  with  a  little  cold  water,  pour  into  100  c.c.  of 
boiling  water,  boil  for  a  minute,  and  cool.  Add  about  2  c.c. 

107 


108  VOLUMETRIC  ANALYSIS 

of  10  per  cent,  potassium  iodide  solution.  Dip  strips  of 
filter  paper  into  the  mixture  and  hang  them  over  a  glass  rod 
until  dry.  Preserve  the  strips  in  a  stoppered  bottle. 

Zinc  by  Means  of  Standard  Sodium  Sulphide. 

The  zinc  is  precipitated  by  means  of  a  standard  sodium 
sulphide  solution,  which  is  added  until  a  slight  excess  can  be 
detected  in  the  solution.  Lead,  copper,  iron,  manganese, 
nickel,  and  similar  metals  must  be  removed  prior  to  the 
titration. 

The  following  solutions  are  required  : — 

Standard  Sodium  Sulphide  Solution. — Dissolve  about  13 
grams  of  sodium  hydroxide  in  about  300  c.c.  of  water. 
Divide  this  solution  into  two  equal  portions,  and  saturate 
one  portion  with  hydrogen  sulphide ;  add  the  other  portion, 
and  dilute  to  one  litre.  This  solution  must  be  standardised 
by  means  of  a  standard  zinc  solution.  As  it  slowly  changes 
in  concentration  on  keeping,  it  must  always  be  standardised 
shortly  before  use. 

Standard  Zinc  Solution. — Dissolve  10-00  grams  of"  puriss  " 
zinc  in  hydrochloric  acid,  and  dilute  to  one  litre.  A  solution 
of  the  same  concentration  may  be  prepared  by  dissolving 
44-00  grams  of  pure  zinc  sulphate  in  water  and  diluting  to 
one  litre. 

Indicator  Solution. — Mix  solutions  of  lead  acetate  and 
Rochelle  salt,  and  add  sodium  hydroxide  until  the  lead 
tartrate  has  redissolved. 

Titration  -with  Sulphide  Solution.  —  To  a  measured 
volume  of  the  zinc  solution,  add  10  c.c.  of  ammonium  car- 
bonate, and  then  sufficient  ammonia  to  redissolve  the 
precipitate.  From  a  burette  add  the  sodium  sulphide  until 
the  following  test  shows  that  there  is  sulphide  in  solution. 

Place  a  drop  of  the  mixture  on  filter  paper  so  that,  on 
expanding,  it  will  reach  a  drop  of  the  lead  indicator.  If 
the  solution  contains  sulphide,  a  dark  line  will  appear  at 
the  boundary.  Zinc  sulphide  will  also  give  a  black  coloration 
with  the  lead  indicator,  and  it  is  therefore  essential  that  the 
test  be  performed  in  such  a  manner  that  the  precipitate  does 
not  reach  the  lead  indicator. 


PART    III 


GRAVIMETRIC   ANALYSIS 

IN  the  gravimetric  method  of  analysis,  the  determination 
of  weight  is  the  principal  quantitative  measurement  involved. 
Although,  as  a  general  rule,  the  procedure  is  less  simple  and 
the  manipulation  more  difficult  than  in  volumetric  analysis, 
this  is  not  necessarily  the  case.  In  order  to  determine,  for 
example,  the  amount  of  zinc  in  a  sample  of  basic  zinc  car- 
bonate, a  weighed  quantity  of  the  substance  is  heated  in  a 
crucible  to  dull  redness  until  it  is  converted  into  zinc  oxide. 
The  weight  of  the  zinc  oxide  is  then  ascertained,  and,  as  the 
composition  of  the  oxide  is  known,  the  percentage  of  zinc 
in  the  basic  carbonate  is  easily  calculated  from  the  weight  of 
the  oxide  obtained.  It  is  obvious,  however,  that  the  result 
will  not  be  correct  unless  the  residue  consists  of  zinc  oxide 
only. 

When  a  complete  analysis  of  a  complex  substance  is 
required,  the  analytical  process  is  less  simple,  and  it  has 
already  been  stated  that,  as  a  rule,  the  constituents  of  the 
substance  must  be  separated  from  one  another  before  the 
amount  of  each  can  be  ascertained.  The  separation  is 
usually  accomplished  by  precipitating  each  constituent  in 
the  form  of  an  "  insoluble  "  compound,  which  is  then  filtered, 
washed,  dried,  and  weighed.  The  attainable  accuracy  of  the 
analysis  depends  mainly  on  the  insolubility  of  the  precipitate, 
on  the  completeness  of  its  separation  from  the  other  substances 
present,  and  on  its  composition  at  the  time  of  weighing  being 
perfectly  definite  and  known. 

One  of  the  most  difficult  problems  met  with  in  quantitative 
analysis  is  the  selection  of  good  methods  of  separation.  A 

109 


110  GRAVIMETRIC  ANALYSIS 

perfect  separation  is  rarely  obtained  by  the  methods  adopted 
in  qualitative  analysis  ;  these  methods  are  often  quite  unsuit- 
able, or  require  modification,  for  the  purposes  of  quantitative 
analysis.  The  separation  of  iron  and  magnesium,  for  example, 
by  means  of  ammonium  chloride  and  ammonia,  is  imperfect. 
All  the  iron  is  precipitated  as  ferric  hydroxide,  but,  even  in 
presence  of  a  large  excess  of  ammonium  chloride,  the  precipi- 
tate always  contains  more  or  less  magnesium  hydroxide.  In 
order  to  separate  the  co-precipitated  magnesium,  the  ferric 
hydroxide,  after  it  has  been  filtered  and  washed,  is  dissolved 
in  acid,  and  ammonium  chloride  and  ammonia  are  again 
added  to  the  solution.  After  filtering  the  ferric  hydroxide, 
it  may  be  necessary  to  dissolve  it  again,  and  to  precipitate  it 
a  third  time  before  the  separation  of  the  iron  and  magnesium 
is  complete.  The  several  filtrates  are  then  combined,  and 
the  magnesium  in  solution  is  precipitated  as  magnesium 
ammonium  phosphate. 

It  is  evident,  from  the  foregoing,  that  precipitation  and 
filtration  form  an  important  part  of  the  routine  of  gravi- 
metric analysis ;  the  general  description  of  these  operations 
is  given  in  Part  I.,  whilst  some  of  the  apparatus  and  processes 
peculiar  to  gravimetric  analysis  are  described  in  the  following 
pages. 

NOTES   ON   APPARATUS. 

(See  also  p.   14) 

Porcelain  Crucibles. — For  most  ordinary  purposes, 
crucibles  of  good  porcelain,  i£  inches  in  diameter  and  I  inch 
high,  are  suitable.  Before  being  exposed  to  a  high 
temperature,  porcelain  crucibles  should  be  carefully  heated 
with  a  small  flame,  in  order  to  avoid  fracture.  Porcelain 
crucibles  must  not  be  used  for  substances  that  are  to  be 
treated  with  hydrofluoric  acid,  or  for  fusions  with  sodium 
hydroxide,  sodium  peroxide,  or  alkali  carbonates ;  and  they 
are  not  so  suitable  as  platinum  crucibles  for  substances  that 
require  heating  to  a  very  high  temperature. 

Silica  Crucibles. — Although  probably  more  fragile  than 
porcelain  crucibles,  silica  crucibles  may  be  safely  exposed  to 
sudden  changes  of  temperature  without  any  risk  of  fracture. 


CARE  OF  PLATINUM  VESSELS  111 

Silica  vessels  must  not  be  used    for  alkalis  or  hydrofluoric 
acid. 

Platinum  Crucibles. — When  the  use  of  a  platinum 
crucible  is  admissible,  it  is  usually  preferable  to  one  of 
porcelain.  Platinum  crucibles  can  be  more  readily  and 
more  uniformly  heated  to  redness  than  porcelain  crucibles. 
On  account  of  the  expense  of  platinum,  however,  it  is  often 
necessary  to  restrict  the  use  of  platinum  vessels  to  cases 
where  they  are  indispensable.  The  following  rules  and 
precautions  with  regard  to  platinum  vessels  must  be  observed : — 

1.  Platinum  crucibles  must  not  be  exposed  to  the  reducing 
area  of  a  flame,  or  to  a  luminous  flame,  as  this  will  cause 
the  metal  to  become  brittle  and  to  lose  its  lustre — owing, 
probably,  to  the  formation  of  a  carbide  of  platinum. 

2.  Compounds  of  lead,  silver,  zinc,  tin,  bismuth,  arsenic, 
and   antimony  must  not   be  heated   in    platinum   crucibles, 
since  reduction   to   the  metallic  state  may  occur,  and  the 
metals,  having  comparatively  low  melting  points,  may  alloy 
with  the  platinum. 

3.  Great  care  should  be  taken  in  igniting  phosphates  in 
platinum  crucibles,  as  the  presence  of  reducing  substances, 
such  as  charred  filter  paper,  may  result  in  the  formation  of 
traces   of  phosphorus  which,  combining  with  the  platinum, 
render  it  brittle.     It  is  safer  to  use  a  porcelain  crucible  for 
phosphates. 

4.  Platinum  crucibles  must  not  be  used  for  fusions  with 
hydroxides  or  nitrates  of  the  alkalis. 

5.  Evaporations  or  fusions  in  which  chlorine,  bromine,  or 
iodine  is  set  free  must  not  be  performed  in  platinum  vessels. 
This   rule  applies  to  mixtures   in  which  both  chloride  and 
nitrate  are  present. 

6.  Platinum   ware   should    be    kept   scrupulously   clean. 
Adhering   substances  or  stains  can  sometimes  be  removed 
by  boiling   a   little   concentrated   hydrochloric   acid   in   the 
crucible,  or  by  fusing  a  little  potassium  bisulphate  in  the 
crucible  and  removing  the  salt  by  means  of  boiling  water. 
The  crucible  should  then  be  polished  with  moist  sea  sand, 
which  is  gently  rubbed  on  the  surface  of  the  metal  with  the 
finger.     After  polishing,  the  platinum  should  be  rinsed  with 
distilled  water  and  dried. 


112 


GRAVIMETRIC  ANALYSIS 


FIG.  29. 
Perforated 


important    to 


The  interior  of  platinum  basins  which  have  been  purposely 
roughened  for  electrolytic  analysis  must  not  be  polished  with 
sand. 

Triangles. — A  very  satisfactory  form  of  pipe-clay  or 
porcelain  triangle,  on  which  a  crucible  is 
placed  during  the  process  of  heating,  is 
shown  in  Fig.  29.  For  the  usual  size  of 
crucible,  a  triangle  with  sides  2j  inches 
long  is  suitable.  Nickel  wire  is  preferable 
to  iron  wire  on  account  of  its  much  greater 
durability. 

Silica  Plates. — It  is  often 
exclude  flame  gases  from  the  interior  of 
a  crucible  during  an  ignition,  and,  for 
this  purpose,  the  device  shown  in  Fig.  30 
may  be  used.  It  consists  of  a  silica 
plate,  5  inches  square,  in  which  is  cut  a 
round  opening  large  enough  to  admit  the 
crucible  to  two-thirds  of  its  depth.  The 
plate  is  held  in  an  inclined  position  by 
means  of  a  clamp.  A  higher  temperature 
in  the  crucible  may  be  reached  by  using 
a  silica  plate  with  a  larger  opening,  and 
placing  over  the  latter  a  disc  of  platinum 
in  which  a  hole  is  cut  to  fit  the  crucible. 

Crucible  Tongs. — Crucible  tongs  should 
be  made  of  brass  or  of  gun-metal,  and  it 
is  an  advantage  to  have  them  fitted  with 
platinum  tips.  They  must  be  kept  scrupulously  clean. 

The  Gooch  Crucible. 

Filtration  by  means  of  a  Gooch  crucible  is  frequently 
advantageous,  and  is  a  most  convenient  method  of  collecting 
a  precipitate  directly  in  the  crucible  in  which  it  is  finally 
weighed.  A  porcelain  Gooch  crucible  of  the  size  and  shape 
shown  in  Fig.  31  is  suitable  for  most  purposes.1  The  bottom 
of  the  crucible  is  perforated  with  a  number  of  small  holes. 


FIG.  30. 


1  Gooch  crucibles  of  the  size  and  shape  shown  in  the  figure  may  be 
obtained  from  Baird  and  Tatlock  (Glasgow). 


THE  GOOCH  CRUCIBLE  113 

A  useful  accessory  is  a  perforated  porcelain  disc  equal  in 
size  to  the  crucible  bottom.  The  crucible,  which  is  always 
used  in  conjunction  with  the  filter-pump,  is  fitted  into  a  glass 
adapter  by  means  of  a  narrow  rubber  ring  (cut  from  a  piece 
of  rubber-tubing,  I  inch  in  diameter),  and  the  adapter  passes 
through  the  rubber  stopper  of  a  filter-flask. 


Gooch  Crucible 

Asbestos 

Perforated   Plate 
-Asbestos 

FlG.  31. — Section  of  Gooch  Crucible  (Actual  Size). 

The  filtering  medium  is  fine,  short-fibre  asbestos,  and  a 
special  quality  is  sold  for  the  purpose.  For  immediate  use, 
i  gram  of  the  asbestos  is  shaken  up  with  200  c.c.  of  water  in 
a  stoppered  bottle.  If  suitable  asbestos  is  not  available,  the 
ordinary  white  fibrous  variety  is  cut  into  pieces  about  J  inch 
long,  any  hard  lumps  being  rejected.  One  gram  of  the  cut 
pieces  is  mixed  with  about  200  c.c.  of  water  in  a  flask,  and  a 
rapid  current  of  air  is  blown  through  the  mixture  in  order 
to  disintegrate  the  fibre.  About  20  c.c.  of  concentrated 
hydrochloric  acid  is  then  added,  and  the  mixture  is  boiled 
for  a  short  time  in  order  to  extract  soluble  matter.  The 
asbestos  is  then  filtered  (using  a  small  Biichner  funnel, 
or  an  ordinary  funnel  provided  with  a  platinum  cone 
but  without  filter  paper),  and  is  thoroughly  washed  with 
warm  water  until  the  filtrate  is  free  from  acid.  The 
asbestos  is  then  mixed  with  200  c.c.  of  water  in  a  stoppered 
bottle. 

Preparation  of  the  Asbestos  Filter. — Connect  the  filter- 
flask,  fitted  with  the  adapter  supporting  the  crucible,  to  the 
filter-pump  (Fig.  32).  In  using  a  Gooch  crucible,  it  is  most 
important  that  the  filtration  should  not  be  conducted  under 
too  great  a  pressure,  and,  to  prevent  this,  the  connection 

H 


114 


GRAVIMETRIC  ANALYSIS 


FIG.  32. 


with  the  pump  should  include  some  means  of  regulating  the 

pressure,  such  as  that  described  later. 

Shake  up  the  asbestos  mixture  and  pour  from  15  to  30 
c.c.  into  the  crucible.  The  larger 
quantity  should  be  used  for  very 
fine  precipitates ;  20  c.c.  of  the 
mixture  gives  a  pad  of  asbestos 
weighing  about  a  centigram,  and, 
as  a  rule,  this  is  sufficient.  Allow 
the  water  to  drain,  without  using 
suction.  Then  start  the  filter-pump, 
using  the  maximum  pressure  the 
regulator  will  allow,  and  carefully 
drop  the  perforated  disc  into  the 
crucible.  Add  2  or  3  c.c.  more  of 
the  asbestos  mixture,  and  then 
wash  the  filter  with  about  100  c.c. 
of  water. 
If  the  filter  is  properly  prepared  with  suitable  asbestos, 

100  c.c.  of  water  will  pass  through  in  less  than  one  minute, 

under  a  pressure  equal    to  that  of   i|   inches   of  mercury. 

Place  the  crucible  on  a  watch-glass 

and    dry    it    for   an    hour    (in    the 

steam-oven  or  air-oven)  at  the  same 

temperature   as    that   required    for 

the  precipitate  that  is  subsequently 

to  be  collected  in  the  crucible.     If 

the    precipitate    requires    ignition, 

place  the   crucible   within  a   larger 

platinum  crucible  (or  a  nickel  cru- 
cible), fitted  with  an  asbestos  ring, 

as  shown  in  Fig.  33,  and  heat  the 

larger  crucible  with  the  flame.     Cool  the  Gooch  crucible  in  a 

desiccator,  and  weigh  it.     It  is  then  ready  for  use. 

Pressure   Regulator  for  Use   with  the  Filter-pump. — 

Some  means  should  always  be  used  of  limiting  the  maximum 

pressure  under  which  filtration  by  means  of  the  filter-pump 

is  conducted,  and  the  following  arrangement  is  simple  and 

effective. 

Three   tubes   pass   through   a   cork    fitted  into   a  bottle 


THE  ROSE  CRUCIBLE  115 

(Fig.  34).     A  and  B  are  connected  to  the  filter-pump  and  to 

the   filter-flask,   respectively.     The   tube  C  is  drawn  out  to 

form   a  capillary  at  its   lower   end  and   forms 

an    air-leak.      The    size    of    the    capillary    is 

adjusted  by  trial,  so  that,  when    the   pump  is 

working  at  full  power,  the  pressure  in  the  bottle  A 

when  B  is  closed  cannot  fall  below  a  definite 

value,  viz.,  about  2  inches  of  mercury  less  than 

atmospheric  pressure.    The  adjustment  is  easily 

made    with    the    help    of    a    simple    pressure 

gauge ;    a   piece   of  glass   tubing   bent   into   a 

U-form  and  containing  mercury  is  suitable  for 

the  purpose. 

The  bottle  also  safeguards  the  contents  of 
the  filter-flask  from  contamination  with  water 
from  the  filter-pump,  should  the  water-pressure  momentarily 
fail. 

The  Rose  Crucible. 

When  it  is  necessary  to  ignite  a  precipitate  in  an 
atmosphere  of  hydrogen,  carbon  dioxide,  or  oxygen,  a 
Rose  crucible  is  used.  The  crucible,  its 
cover,  and  the  tube  through  which  the  gas 
is  led  into  the  crucible  (all  of  porcelain  or 
silica),  are  shown  in  Fig.  35. 

The  crucible  and    lid  should   always   be 
FIG  35          weighed  separately  as  the  lid  often  breaks 

during  the  ignition. 
Hydrogen  should  be  prepared  in  a  small  Kipp  generator 
from  sulphuric  acid  (6  N)  and  arsenic-free  zinc.  A  little 
copper  sulphate  solution  should  be  poured  over  the  zinc 
before  the  generator  is  charged  with  acid.  The  gas  must 
be  purified  and  dried  by  passing  it  through  two  wash- 
bottles — the  first  containing  potassium  permanganate  solu- 
tion acidified  with  dilute  sulphuric  acid,  and  the  second 
containing  concentrated  sulphuric  acid.  Before  the  hydrogen 
is  used,  it  must  be  tested  and  proved  free  from  air  by 
collecting  a  sample  of  the  gas  in  a  test-tube  by  displace- 
ment of  air,  and  noting  whether  it  burns  quietly  when 
ignited. 


116  GRAVIMETRIC  ANALYSIS 

Carbon  Dioxide^  prepared  in  a  Kipp  generator  from 
marble  and  hydrochloric  acid,  must  be  washed  and  dried 
by  passing  it  through  a  mixture  of  water  and  sodium 
bicarbonate,  and  then  through  concentrated  sulphuric 
acid. 

Oxygen,  supplied  from  a  gas-holder  (filled  from  a  cylinder 
of  the  compressed  gas),  should  be  dried  by  passing  it 
through  concentrated  sulphuric  acid. 

In  all  cases,  the  flow  of  gas  is  best  regulated  by  means  of 
a  tap  or  a  screw-clip  placed  on  the  rubber  connection  between 
the  sulphuric  acid  wash-bottle  and  the  crucible. 

THE  IGNITION  AND  WEIGHING  OF  PRECIPITATES. 

After  a  precipitate  has  been  filtered  and  washed,  it  re- 
quires further  treatment  before  it  can  be  weighed.  In  the  first 
place,  if  a  filter  paper  has  been  used,  it  must  be  destroyed  by 
incinerating  it  either  in  presence  of  the  whole  precipitate  or 
after  separating  the  precipitate  from  it.  The  precipitate, 
together  with  the  filter  ash,  is  then  "  ignited  "  in  a  weighed 
crucible.  (The  terms,  "  ignite "  and  "  ignition,"  are  com- 
monly used  in  analytical  chemistry,  and  refer  to  the 
process  of  heating  a  substance  to  a  high  temperature, 
without  allowing  the  direct  access  of  the  flame  to  the 
substance.)  The  purpose  of  the  ignition  is  (i)  to  dry  or 
dehydrate  the  precipitate  completely,  and,  in  many  cases, 
(2)  to  convert  the  precipitate,  which  may  be  of  uncertain 
composition,  into  another  compound  of  definite  and  known 
composition.  Copper,  for  example,  may  be  precipitated  as 
hydrated  copper  oxide,  CuO,  #H2O,  which  is  ignited  and 
weighed  as  anhydrous  cupric  oxide,  CuO ;  and  zinc  may  be 
precipitated  as  basic  carbonate  which  is  of  variable  composi- 
tion but  is  easily  converted  by  ignition  into  zinc  oxide,  ZnO. 
The  weight  of  the  ignited  precipitate  is  then  ascertained  by 
weighing  the  crucible  and  its  contents,  and  deducting  the 
weight  of  the  crucible  and  that  of  the  filter  ash  (unless  the 
latter  is  negligible)  from  the  total  weight. 

The  ignition  is  performed  by  heating  the  crucible  con- 
taining the  precipitate  with  the  flame  of  a  Bunsen  burner, 


IGNITION  OF  PRECIPITATES  117 

a  Meker  burner,  or  a'blowpipe,  according  to  the  temperature 
required. 

The  Bunsen  Burner. — A  good  Bunsen  burner,  giving  a 
flame  of  medium  size,  should  be  used  for  heating  purposes. 
In  order  that  it  may  be  possible  to  obtain  suitable  non- 
luminous  flames  of  different  sizes,  it  is  most  important  that 
the  air-regulator  of  the  burner  should  be  in  working  order ; 
it  seldom  happens  that  the  mere  lighting  of  the  burner, 
without  carefully  adjusting  the  air  supply,  gives  the  best 
flame  for  a  given  purpose. 

When  a  Bunsen  burner  is  used  for  strongly  heating  a 
crucible,  rather  more  air  than  is  just  required  to  produce  a 


FIG.  36.  FIG.  37. 

Incorrect  Position  Correct  position 

of   Crucible   in  of  Crucible  in 

Bunsen  flame.  Bunsen  flame. 

non-luminous  flame  should  be  admitted  by  means  of  the 
regulator ;  too  much  air,  however,  gives  a  noisy  flame  which 
is  unsuitable.  The  position  of  a  crucible  in  a  Bunsen  flame 
is  important.  If  the  crucible  is  placed  so  near  the  burner 
that  the  inner  cone  of  unburnt  gas  impinges  on  it  (Fig.  36), 
the  bottom  of  the  crucible  will  not  become  properly  heated  ; 
and  the  crucible  must  not  be  enveloped  in  a  large  flame 
burning  with  a  restricted  air  supply.  The  proper  position, 
in  which  the  bottom  of  the  crucible  is  about  half  an  inch 
above  the  top  of  the  inner  cone,  is  shown  diagrammatically 
in  Fig.  37,  and,  in  order  that  the  position  of  the  crucible  in 
the  flame  may  be  easily  adjusted,  the  pipe-clay  triangle  on 
which  the  crucible  rests  should  be  supported  on  a  moveable 
retort-stand  ring. 

The   Meker  Burner. — When   a   Bunsen    flame    is   fully 


118 


GRAVIMETRIC  ANALYSIS 


aerated,  the  volume  of  air  passing  into-  the  burner  is  about 

2.5  times  the  volume  of  the  gas,  whereas  for  the  complete 
combustion  of  one  volume  of  coal-gas  about  six 
volumes  of  air  are  required.  If  a  mixture 
of  gas  and  air  in  the  latter  proportions  were 
lighted  at  an  ordinary  burner,  the  flame  would 
"  strike  back  "  and  burn  at  the  bottom  of  the 
tube. 

In   the    Meker    burner,   the    holes    for    the 
admission    of    air   are    large   enough    to    pass 
FIG.  38.       sufficient   air   for   the   complete  combustion  of 

Correct  Position  the  gas,  and  a  nickel  grid  is  fitted  into  the  top 

of  Crucible  in  of  the   burner   in   order   to  prevent  the  flame 

me>    striking  back  (Fig.  39).     The  flame  of  a  Meker 

burner  is  smaller,  and  therefore  hotter,  than  a  Bunsen  flame 

burning  the   same  amount  of  gas,  and   the  cold   centre  of 

unburnt  gas  is  entirely  absent.     The 

hottest  part  of  the  flame  is  close  to 

the  nickel  grid,  but  the  temperature 

of  the  flame  is  much  more  uniform 

than  that  of  a    Bunsen  flame.     For 

igniting    precipitates     it    is    seldom 

necessary   to    use    a    blowpipe    if   a 

Meker  burner  is  available. 

Drying  the  Precipitate  and  the 
Filter. 

It  is  often  necessary  to  dry  the 
precipitate  and  the  filter  before  the 
latter  is  incinerated.  To  do  this, 
first  remove  the  water  in  the  stem  of 
the  funnel  by  means  of  filter  paper ; 
cover  the  mouth  of  the  funnel  with  a  piece  of  paper,  the 
latter  being  folded  over  the  rim  of  the  funnel  so  that  each 
fold  overlaps  the  preceding  one ;  and  then  place  the  funnel 
in  the  steam-oven  in  an  upright  position,  and  leave  it  there 
for  several  hours  until  the  precipitate  and  paper  are  dry. 


FIG.  39. 


INCINERATION  OF  THE  FILTER  119 


Incineration  of  the  Filter. 

In  certain  cases,  the  filter  may  be  incinerated  in  presence 
of  the  whole  precipitate ;  in  others,  the  precipitate  must  be 
detached  as  far  as  practicable  from  the  filter  before  the  latter 
is  incinerated.  The  procedure  depends  on  the  nature  of  the 
precipitate. 

The  filter  must  be  incinerated  apart  from  the  precipitate 
(i)  if  the  precipitate  is  fusible  at  the  temperature  of  incinera- 
tion, e.g.,  silver  chloride,  or  (2)  if  the  precipitate  suffers 
reduction  to  the  metallic  state  during  the  charring  of  the 
filter  paper,  e.g.,  silver  chloride,  lead  sulphate,  zinc  carbonate, 
or  (3)  if  the  compound  which  is  to  be  weighed  is  decomposed 
at  the  high  temperature  of  the  incineration,  e.g.,  if  calcium 
oxalate  is  to  be  converted  into  calcium  carbonate,  it  must 
not  be  heated  above  dull  redness. 

The  filter  may  be  incinerated  in  presence  of  the  precipitate 
in  the  case  of  (i)  silica;  (2)  the  oxides  of  iron,  aluminium, 
chromium,  and  manganese  ;  and  (3)  the  sulphates  of  barium, 
strontium,  and  calcium. 

The  incineration  is  performed  in  a  porcelain  crucible,  or 
in  a  platinum  crucible  if  the  use  of  the  latter  is  permissible 
(see  p.  in). 

Tare  of  the  Crucible. — Place  the  clean  crucible,  covered 
with  the  lid,  on  a  pipe-clay  triangle,  and  heat  it  to  redness 
for  a  few  minutes  with  a  properly  adjusted  Bunsen  flame. 
Remove  the  flame,  and  after  about  a  minute  lift  the  lid  of 
the  crucible  with  tongs  and  place  it  temporarily  on  the 
desiccator  cover,  which  is  held  inverted  in  the  left  hand ; 
then  remove  the  crucible  and  finally  the  lid  to  the  desiccator. 
(Except  on  the  ground  rim,  the  desiccator  cover  must,  of 
course,  be  free  from  grease.)  Allow  the  desiccator  to  remain 
in  the  balance-room  for  twenty  to  thirty  minutes,  then  weigh 
the  crucible  and  lid,  and  afterwards  replace  them  in  the 
desiccator.  After  the  crucible  has  been  heated  and  weighed, 
it  must  not  be  placed  directly  on  the  bench,  but  only  on  the 
pipe-clay  triangle,  in  the  desiccator,  or  (when  cold)  on  a  sheet 
of  clean  paper. 


120 


GRAVIMETRIC  ANALYSIS 


Incineration  of  the  Filter  in  Presence  of  the  Precipitate. 

When  this  method  is  applicable,  it  is  not  necessary  to  dry 
the  precipitate  in  the  steam-oven ;  but  large  precipitates  of 
chromic,  ferric,  and  aluminium  hydroxides  may  be  partially 
dried  as  described  on  p.  118,  before  proceeding  with  the 
incineration. 

Detach  the  filter  very  carefully  from  the  funnel  by  means 
of  a  small  spatula,  and  remove  it  from  the  funnel.  Fold  the 
filter  paper  so  as  to  form  a  small  packet  enclosing  the  pre- 
cipitate, care  being  taken  not 
to  tear  the  paper.  Place  the 
packet  in  the  weighed  crucible 
and  press  it  down  gently. 
Remove  any  trace  of  the 
precipitate  adhering  to  the 
funnel  with  a  piece  of  "  ash- 
less  "  filter  paper — first  moist- 
ening the  funnel,  if  necessary, 
by  breathing  into  it  —  and 
drop  this  piece  also  into  the 
crucible.  Place  the  crucible 
on  a  pipe-clay  triangle  in  a 
slanting  position  and  cover  it 
partially  with  the  lid  ;  the  lid 
should  rest  partly  on  the 
triangle,  as  shown  in  Fig.  40, 
and  a  piece  of  platinum  foil 
should  be  wrapped  round  the 
wire  of  the  triangle  at  the 
point  of  contact  with  the  lid. 
Place  a  small  (f  inch)  flame  under  the  crucible  lid  and 
about  i  inch  from  it,  as  shown  in  the  figure.  The  hot  gases 
are  deflected  into  the  crucible,  and  the  contents  soon  become 
dry.  When  the  paper  begins  to  char,  remove  the  lid  of  the 
crucible  and  place  it,  meanwhile,  on  a  glazed  tile  or  a  watch- 
glass.  Place  the  crucible  upright,  and  adjust  the  size  of  the 
flame  and  the  distance  of  the  crucible  from  it,  so  that  the 
paper  gradually  chars  without  taking  fire.  If  the  paper 
should  take  fire,  remove  the  flame  and  cover  the  crucible  with 


FIG.  40. 


INCINERATION  OF  THE  FILTER  121 

the  lid  for  a  moment.  When  the  escape  of  vapour  ceases 
and  the  charring  of  the  paper  is  complete,  heat  the  crucible 
more  strongly  until  all  the  carbon  is  oxidised.  If  the  carbon 
burns  with  difficulty,  the  oxidation  may  be  accelerated  by 
holding  the  crucible  lid  with  tongs  in  various  oblique  positions 
over  the  mouth  of  the  crucible.  A  deposit  of  carbon  is  some- 
times found  on  the  under-surface  of  the  crucible  lid,  but  this 
is  easily  removed  by  heating  the  inverted  lid  with  the  flame. 

The  precipitate  is  now  ready  for  ignition,  and  the  special 
instructions  given  for  each  case  should  be  followed.  After 
igniting,  remove  the  flame,  wait  for  about  a  minute,  then 
place  the  crucible  in  a  desiccator  and  allow  it  to  cool  for 
twenty  or  thirty  minutes.  Weigh  the  crucible  with  its  contents. 

In  all  cases,  the  ignition  must  be  repeated  as  often  as 
may  be  necessary  until  constant  weight  is  attained.  By 
"constant  weight"  is  here  meant  that  the  difference  be- 
tween two  consecutive  weights  is  not  more  than  two-tenths 
of  a  milligram,  z>.,  0-0002  gram.  The  weighing  must  be  per- 
formed as  quickly  as  possible,  especially  if  the  precipitate  is 
hygroscopic,  and,  when  a  weighing  is  repeated,  all  the  weights 
used  in  the  previous  weighing  should  be  placed  on  the  balance 
before  the  crucible  is  removed  from  the  desiccator.  It  is  a 
mistake  to  leave  an  ignited  precipitate  in  a  desiccator  for  a 
long  time,  e.g.,  overnight,  before  weighing  it ;  it  should  be  re- 
ignited,  and  then  weighed  after  cooling  for  not  more  than 
about  thirty  minutes. 

Incineration  of  the  Filter  apart  from  the  Precipitate. 

The  precipitate  and  filter  must  be  thoroughly  dried  in  the 
steam-oven.  Two  sheets  (about  9  inches  square)  of  glazed 
paper — white  if  the  precipitate  is  coloured,  and  black  if  the 
precipitate  is  white — are  laid  on  the  bench.  A  shallow, 
porcelain  basin,  about  2\  inches  in  diameter,  or  a  watch-glass 
of  the  same  size,  is  placed  on  one  of  the  sheets.  The  Bunsen 
burner  or  other  metal  apparatus,  from  which  particles  of  rust 
or  dirt  are  liable  to  drop,  must  not  on  any  account  be  placed 
on  the  glazed  paper. 

Remove  the  well-dried  filter  from  the  funnel.  Hold  the 
filter  over  the  glazed  paper  and  loosen  the  precipitate  by 
gently  pressing  the  cone-shaped  filter  with  the  fingers. 


122 


GRAVIMETRIC  ANALYSIS 


Transfer  the  bulk  of  the  precipitate  very  carefully  to  the 
basin  or  watch-glass.  Carefully  unfold  the  filter  and  loosen 
the  precipitate  still  adhering  to  it  by  lightly  rubbing  with  the 
paper  itself,  care  being  taken,  however,  not  to  rub  off  any 
paper  fluff.  Empty  this  portion  of  the  precipitate  into  the 
basin,  then  place  the  latter  on  the  second  sheet  of  paper,  and 
cover  the  basin  meanwhile  with  a  clock-glass. 


FIG.  41. 

Fold  the  filter  paper  into  a  narrow  strip,  as  shown  in  A 
to  D  (Fig.  41),  the  shaded  portion  in  A  representing  the 
soiled  part  of  the  paper.  Carefully  wipe  off  any  traces  of  the 
precipitate  adhering  to  the  funnel  by  means  of  the  paper 
strip — first  breathing  into  the  funnel  in  order  to  moisten  it — 
and  then  wrap  up  the  strip  into  a  compact  roll,  as  shown  in 
E  and  F.  Place  the  roll  in  the  weighed  crucible  which, 
meanwhile,  is  left  in  the  desiccator. 

If  any  precipitate  has  been  allowed  to  fall  on  the  glazed 
paper,  transfer  it  very  carefully  to  the  basin  by  bending,  but 
not  folding,  the  sheet,  and  sweeping  the  particles  into  the 
basin  by  means  of  a  small  brush. 

Now  place  the  open  crucible  containing  the  filter  on  a 
pipe-clay  triangle,  and  incinerate  the  filter  in  the  manner 
described  on  p.  120.  Allow  the  crucible  to  cool,  and  then  treat 
the  filter  ash  in  accordance  with  the  instructions  given  for  the 
particular  case  (see,  for  example,  silver  chloride  or  copper 
oxide).  When  the  crucible  is  cold  again,  place  it  on  the 
glazed  paper,  and  very  carefully  transfer  the  main  part  of  the 
precipitate  to  the  crucible.  Finally  ignite  the  precipitate  in 
the  manner  specified  for  each  case,  and  repeat  the  ignition 
until  constant  weight  is  attained. 


Typical  Gravimetric   Exercises 

The  following  section  contains  a  number  of  simple  exer- 
cises which  have  been  so  selected  that  they  involve  all  the 
more  important  manipulative  operations  of  gravimetric 
analysis.  In  each  case  the  exercise  can  be  performed  with 
a  salt  which  is  readily  obtained  in  a  state  of  purity ;  the 
experimental  result  can  therefore  be  checked  by  calculation. 
The  value  of  the  exercises  as  a  preliminary  training  in 
gravimetric  analysis  is  considerably  greater  if  all  the  analyses 
are  performed  with  solutions  or  solids  of  which  the  composi- 
tion is  unknown  to  the  student.  A  list  of  solutions  which  are 
suitable  for  this  purpose,  with  particulars  as  to  their  prepara- 
tion, is  given  in  the  Appendix. 

The  beginner  should  carry  out  most  of  the  exercises 
given  in  this  section  before  proceeding  to  the  analyses 
described  in  later  sections  of  the  book.  The  exercises  are 
arranged  roughly  in  order  of  difficulty,  except  that,  for  con- 
venience in  description,  the  determination  of  aluminium  has 
been  placed  immediately  after  that  of  iron,  although  it 
presents  more  difficulty  than  many  of  the  other  exercises. 

Determination   of  Water  in   Magnesium   Sulphate 
Heptahydrate. 

OUTLINE  OF  METHOD. — A  weighed  quantity  of  the  magnesium  sulphate 
is  heated  to  dull  redness,  and  the  loss  of  weight,  which  represents 
the  water,  is  ascertained. 

Procedure. — Heat  a  crucible  (and  lid)  with  a  full  Bunsen 
flame  for  five  minutes.  Remove  the  flame,  allow  the  crucible 
to  cool  for  about  a  minute,  and  then  place  it  in  a  desiccator 
for  half  an  hour.  Weigh  the  crucible  and  lid  accurately. 
Place  about  0-6  gram  of  magnesium  sulphate,  MgSO4,  7H2O, 
in  the  crucible,  and  weigh  again.  Place  the  covered  crucible, 


124  GRAVIMETRIC  ANALYSIS 

resting  on  a  pipe-clay  triangle,  about  6  inches  above  a  small 
flame  (not  more  than  I  inch  high).  At  intervals  of  a  few 
minutes,  lower  the  crucible  and  increase  the  flame  gradually, 
until  the  bottom  of  the  crucible  is  heated  to  dull  redness. 
Maintain  the  crucible  at  this  temperature  for  about  ten 
minutes.  Allow  the  crucible  to  cool  in  a  desiccator  for 
half  an  hour,  and  weigh.  Repeat  the  heating  process  until 
constant  weight  is  attained. 

From  the  loss  of  weight,  calculate  the  percentage  of  water 
in  the  magnesium  sulphate  heptahydrate. 

Determination  of  Water  in  Barium  Chloride  Crystals. 

Weigh  accurately  in  a  tared  porcelain  crucible  1-5  to  2-0 
grams  of  barium  chloride  crystals.  Heat  the  crucible  and 
contents  to  a  temperature  not  exceeding  dull  redness  until 
constant  weight  is  attained  (compare  previous  exercise). 
From  the  loss  of  weight,  calculate  the  percentage  of  water 
in  the  barium  chloride  crystals. 

Determination  of  Anhydrous  Disodium  Hydrogen 
Phosphate  in  the  Crystalline  Salt. 

In  a  tared  porcelain  crucible  weigh  accurately  about  0-5 
gram  of  sodium  phosphate  (select  crystals  free  from  efflores- 
cence). Heat  the  crucible  and  contents  for  about  an  hour  in 
the  steam-oven,  and  then  at  a  gradually  increasing  tempera- 
ture with  a  Bunsen  flame.  Finally  ignite  at  a  red  heat  for 
about  ten  minutes.  Cool  and  weigh.  Repeat  the  ignition 
until  constant  weight  is  attained.  The  residue  is  sodium 
pyrophosphate, 

2Na2HPO4  =  Na4P2O7 


From  the  weight  of  sodium  pyrophosphate  obtained,  calculate 
the  percentage  of  anhydrous  disodium  hydrogen  phosphate 
in  the  original  crystals. 


ANALYSIS  BY  IGNITION  125 

Determination  of  the  Iron  in  Ammonium  Iron  Alum. 

OUTLINE  OF  METHOD. — The  salt  is  converted  into  ferric  oxide  by  heat- 
ing in  a  crucible,  and  the  percentage  of  iron  in  the  alum  is  calculated 
from  the  weight  of  the  oxide  obtained. 

This  method  is  only  applicable  in  those  cases  where  the 
residue  left  after  ignition  consists  of  pure  ferric  oxide.  It  is 
not  to  be  used,  therefore,  if  non-volatile  impurities  are  likely 
to  be  present. 

Procedure. — Weigh  into  a  tared  crucible  1-5  to  2-0  grams 
of  ammonium  iron  alum.  Heat  the  covered  crucible  and 
contents  very  gently  over  a  small  flame ;  gradually  increase 
the  temperature  until  full  redness  is  attained,  and  continue 
the  heating  for  fifteen  minutes.  Cool  for  one  minute,  transfer 
to  the  desiccator  for  twenty  to  thirty  minutes,  and  weigh. 
Repeat  the  heating  and  weighing  until  the  weight  is 
constant. 

The  result  of  the  experiment  should  be  recorded  as 
follows : — 

Weight  of  crucible  and  alum    .         10-6842 
Tare  of  crucible  and  lid    .         .  9-1722 

Weight  of  alum  taken     -,_  .      .-          1-5120 
Weight  of  crucible  and  Fe2O3  .  9-4241 


94234 
Weight  of  Fe2O3  obtained        .  0-2512 

1S9'7  grams  Fe2O3  represents  1117  grams  Fe,  and   the 
percentage  of  iron  in  the  alum  is  therefore 

0-2512  X  III-7X  IOO 

— =  1 1 -6 1 

J597X  1-5120 

Percentage  of  iron  required  by  the  formula  =  11-58 

Difference  =  +0-03 

The  percentage  error  is  4-0-26.     This  corresponds  to  0-6 
mgrm.  of  ferric  oxide. 


126  GRAVIMETRIC  ANALYSIS 

Other  Examples  of  Analysis  by  Ignition. 

Many  other  determinations  may  be  carried  out  in  the 
manner  described  in  the  last  exercise.  The  method  is 
only  applicable  to  the  determination  of  one  constituent,  and 
then  only  in  those  cases  where  the  residue  left  after  ignition 
consists  of  a  pure  substance,  such  as  a  pure  oxide.  A 
determination  by  this  method  offers  less  experimental  diffi- 
culty and  is  more  expeditious  than  by  a  precipitation 
method. 

The  residue  left  on  ignition  is,  in  most  cases,  an  oxide. 
Before  proceeding  to  the  analysis,  study  the  properties  of  the 
oxide  to  ascertain  to  what  extent  it  may  safely  be  heated ; 
as  a  rule,  this  information  may  be  obtained  by  reference  to 
Part  IV.  of  this  book. 

The  following  are  typical  cases  in  which  this  method  may 
be  used  : — 

Aluminium   in   ammonia   alum.     The    residue   left  after 

ignition  is  the  oxide,  A12O3. 
Barium  in  barium   acetate,  peroxide,  and  nitrate.     The 

residue  left  after  ignition  is  barium  oxide,  BaO. 
Bismuth    in    bismuth    oxynitrate   and    carbonate.     The 

residue  left  after  ignition  is  bismuth  oxide,  Bi2O3. 
Calcium   in   calcium    acetate,  hydroxide,  carbonate,   and 

nitrate.     The  residue  left  after   ignition  is  calcium 

oxide,  CaO. 
Copper  in  copper  hydroxide,  carbonate,  and  nitrate.     The 

residue  left  after  ignition  is  cupric  oxide,  CuO. 
Lead  in  lead  hydroxide,  peroxide,  carbonate,  and  nitrate. 

The  residue  left  after  ignition  is  lead  monoxide,  PbO. 

It  is  often  possible,  by  slight  modification  of  the  procedure, 
to  apply  this  method  to  other  salts,  e.g. — 

(1)  The   iron    in    ferrous   ammonium    sulphate   may   be 

determined  by  oxidation  of  a  weighed  sample  with 
concentrated  nitric  acid  and  subsequent  ignition. 
The  residue  obtained  is  ferric  oxide. 

(2)  Most  sulphates  are  completely  converted  into  oxides 

by  repeated  ignition,  with  addition  of  a  few  pieces  of 
solid  ammonium  carbonate  before  each  ignition. 


IRON  AS  FERRIC  OXIDE  127 


Determination  of  Iron  as  Ferric  Oxide. 

OUTLINE  OF  METHOD. — The  iron,  after  oxidation  to  the  ferric  state  if 
this  should  be  necessary,  is  precipitated  as  ferric  hydroxide  by 
adding  ammonia.  The  precipitate  is  filtered  and  washed.  The 
filter  is  incinerated  together  with  the  precipitate,  and  the  latter 
is  converted  into  ferric  oxide  and  weighed  as  Fe2O3. 

Ferric  Hydroxide  is  a  reddish-brown,  flocculent  precipi- 
tate, practically  insoluble  in  water,  in  dilute  alkalis,  and 
in  ammonium  salts,  but  readily  soluble  in  acids.  In  order  to 
obtain  it  free  from  basic  salt,  it  should  be  precipitated  by 
rapidly  adding  a  moderate  excess  of  ammonia  to  a  cold  or 
warm  (but  not  boiling)  solution,  the  latter  being  continuously 
stirred.  If  washed  free  from  the  last  trace  of  soluble  salts, 
ferric  hydroxide  occasionally  passes  through  the  filter  in 
the  form  of  a  brown  colloidal  solution. 

Ferric  Oxide,  obtained  by  strongly  heating  the  hydroxide, 
is  reddish-brown  or  almost  black  in  colour,  according  to  the 
temperature  of  the  ignition.  Contact  with  a  reducing  flame 
converts  it  partially  into  Fe3O4,  or  even  into  metallic  iron. 
If  ferric  oxide  is  ignited  with  ammonium  chloride,  ferric 
chloride  volatilises.  The  ignited  oxide  dissolves  very  slowly 
in  concentrated  hydrochloric  acid. 

Exercise. — Weigh  accurately,  in  a  scoop  or  watch-glass, 
about  1-3  grams  of  ammonium  ferric  sulphate  (ammonium 
iron  alum).  Transfer  it  to  a  400  c.c.  Jena  glass  beaker, 
provided  with  a  suitable  clock-glass  and  stirring-rod  (see 
Fig.  7,  p.  21).  Dissolve  the  salt  in  water,  add  5  c.c.  of  dilute 
sulphuric  acid,  and  determine  the  iron  as  follows. 

Procedure. — Dilute  to  about  150  c.c.,  warm  the  solution, 
and  precipitate  the  iron  as  ferric  hydroxide  by  rapidly  adding 
a  moderate  excess  of  ammonia  (15  to  20  c.c.  of  2N  solution), 
the  solution  meanwhile  being  continuously  stirred.  Leave 
the  stirring-rod  in  the  beaker,  cover  the  beaker  with  the 
clock-glass,  and  heat  the  contents  until  boiling.  Boil  for 
about  one  minute,  and  make  sure  that  ammonia  is  present 
in  the  escaping  steam.  Remove  the  flame,  place  the  beaker 
on  a  paper  mat,  rinse  the  under  side  of  the  clock-glass  with 
hot  water,  and  allow  the  precipitate  to  settle. 

Before   commencing   to   filter   the   precipitate,   read    the 


128  GRAVIMETRIC  ANALYSIS 

general  instructions  regarding  the  filtration  and  washing  of 
precipitates  given  on  pp.  23  to  27. 

Fit  a  2f -inch  (7  cm.)  funnel  with  an  1 1  cm.  paper.  For 
this  precipitate  the  type  of  funnel  shown  in  Fig.  13  on  p.  27, 
and  the  "  black  ribbon "  variety  of  filter  paper  (p.  24,  foot- 
note) are  preferable. 

Begin  the  filtration  by  decanting  as  much  as  possible 
of  the  clear  liquid  into  the  filter  without  disturbing  the 
precipitate ;  pour  the  liquid  down  the  stirring-rod,  the  latter 
being  held  against  the  rim  of  the  beaker,  and  direct  the  liquid 
against  the  side  of  the  filter  and  not  into  the  apex  (p.  25). 
Do  not  fill  the  filter  quite  to  the  brim.  Replace  the  beaker  on 
the  paper  mat,  add  about  80  c.c.  of  hot  water — pour  the  water 
against  the  side  of  the  beaker  in  order  to  avoid  loss  by  splash- 
ing— and  stir  well.  Allow  the  precipitate  to  settle,  and  once 
more  decant  the  clear  liquid  into  the  filter.  Repeat  this 
process  three  times. 

Now  transfer  the  precipitate  to  the  filter  by  pouring  as 
much  of  it  as  possible  into  the  latter,  and,  by  means  of  a  jet 
of  hot  water,  washing  the  remainder  into  the  filter  in  the 
manner  described  on  p.  25.  Remove  any  traces  of  precipi- 
tate adhering  to  the  beaker  and  stirring-rod  by  rubbing 
with  a  closely  trimmed  feather,  afterwards  rinsing  first  the 
feather,  and  finally  the  beaker  and  the  stirring-rod  once  more. 

Any  precipitate  that  cannot  be  removed  in  this  way  must 
be  dissolved  in  dilute  nitric  acid  (2  to  3  drops  mixed  with  I  c.c. 
of  hot  water),  which  is  brought  into  contact  with  the  entire 
surface  of  the  beaker  by  means  of  the  stirring-rod ;  a  few 
drops  of  ammonia  are  then  added  in  order  to  reprecipitate 
the  ferric  hydroxide,  and  the  minute  precipitate  is  collected 
in  a  separate  small  filter  which  is  then  carefully  washed. 

Finally,  carefully  scrutinise  the  beaker  in  a  good  light  in 
order  to  make  sure  that  no  trace  of  precipitate  remains. 

The  precipitate  and  filter  paper  must  now  be  thoroughly 
washed  with  hot  water  in  the  following  manner: — (i)  Direct 
a  fine  stream  of  water  against  the  filter  paper — blowing  gently 
at  first  in  order  that  the  impact  of  the  water-jet  will  not  cause 
a  portion  of  the  precipitate  to  be  projected  out  of  the  funnel 
— and  then,  with  a  rotary  motion  of  the  wash-bottle  jet,  wash 
the  precipitate  as  far  as  possible  into  the  lower  part  of  the 


IRON  AS  OXIDE  129 

filter.  Allow  the  filter  to  drain  completely,  and  repeat.  (2) 
Direct  the  water-jet  round  the  margin  of  the  filter  paper — 
which  must  be  washed  with  great  care — and  then  into  the 
mass  of  the  precipitate,  which  should  be  well  churned  up  in 
the  operation.  Allow  to  drain,  and  repeat  the  washing  until 
the  filtrate  is  found  to  be  free  from  sulphate.  In  order  to 
test  for  sulphate,  rinse  the  stem  of  the  funnel  with  water,  and 
collect  about  5  c.c.  of  the  filtrate  in  a  test-tube ;  add  a  few 
drops  of  barium  nitrate,  and  warm.  When  no  turbidity  is 
observed,  the  washing  is  complete. 

The  filtration  may  be  carried  out  with  the  help  of  the 
filter-pump.  Gentle  suction  only  should  be  used,  and  the 
pressure  regulated  by  means  of  a  capillary  leak,  as  described 
on  p.  1 14. 

While  the  filtration  is  in  progress,  a  clean  crucible 
(porcelain  or  platinum)  is  ignited  at  a  red  heat,  cooled  in 
a  desiccator  for  thirty  minutes,  and  weighed.  The  filter, 
together  with  the  precipitate,  is  then  incinerated  without 
previous  drying,  in  the  manner  described  on  p.  120.  When 
the  incineration  is  complete,  ignite  the  ferric  oxide  with  a 
full  Bunsen  flame  in  the  partially  covered  crucible  for  ten 
minutes,  cool  in  a  desiccator  for  thirty  minutes,  and  weigh. 
Repeat  the  ignition  until  constant  weight  is  attained. 

From  the  weight  of  Fe2O3  obtained,  calculate  the  percent- 
age of  iron  in  ammonium  iron  alum. 

The  following  example  shows  how  the  weighings  should 
be  recorded,  the  result  calculated,  and  the  error  stated  : — 

Weight  of  scoop  +  iron  alum  .         .        .     =     6-8244 
Weight  of  scoop =     5-4474 

Weight  of  iron  alum  =     1-3770 

Weight  of  crucible  =  15-2816 

Weight  of  crucible +  Fe2O3  ( ist  ignition)  =  15-5107 

„       (2nd  ignition)  =  15-5102 

(3rd  ignition)  =  15-5103 

Weight  of  Fe2O3    .        ;        .        .      ., ..    =  0-2287 

159-7  grams  Fe2O3  =  111-7  grams  Fe. 
0-2287  gram  Fe2O3  =  0-2287  x  gram  Fe. 

I 


130  GRAVIMETRIC  ANALYSIS 

1 1 1-7          100 
Percentage  of  iron  found  =0-2287    x    JT^    x   7^77 

=  11-61 

Percentage  of  iron  calculated 
from  the   formula  of  iron 
alum         .        ,        .         .     =11-58 
Difference     .        .        .     =  +0-03 
Error  (3  in  1 1 60)  .         .     =  +0-26  per  cent. 

=  +0-6  mgrm.  of  Fe2O3. 

Determination  of  Aluminium  as  Oxide. 

OUTLINE  OF  METHOD. — The  aluminium  is  precipitated  as  aluminium 
hydroxide  by  means  of  ammonia  in  presence  of  ammonium 
chloride.  The  precipitate  is  converted  into  the  oxide  by  ignition, 
and  is  weighed  as  A12O3. 

Aluminium  Hydroxide  is  a  bulky,  gelatinous  precipitate, 
slightly  soluble  in  ammonia,  but  almost  insoluble  in  ammonia 
containing  ammonium  salts.  Freshly  precipitated  aluminium 
hydroxide  dissolves  readily  in  dilute  acids,  but  after  keeping 
for  some  time  it  becomes  almost  insoluble.  It  is  converted 
into  alumina  by  ignition  ;  a  very  high  temperature  is  required 
for  complete  dehydration. 

Aluminium  Oxide  (Alumina],  obtained  from  the  hydroxide 
by  ignition,  dissolves  very  slowly  in  hot  concentrated  hydro- 
chloric acid.  It  may  be  brought  into  solution  more  easily 
by  fusion  with  potassium  hydrogen  sulphate.  It  is  not 
decomposed  or  volatilised  at  the  highest  temperature 
attainable  with  a  blowpipe  flame. 

Exercise. — Weigh  accurately,  in  a  scoop  or  a  watch- 
glass,  about  1-8  grams  of  ammonium  aluminium  sulphate, 
(NH4)2SO4,  A12(SOJ3,24H2O.  Transfer  it  to  a  400  c.c.  Jena 
glass  beaker  provided  with  a  clock-glass  cover  and  stirring- 
rod.  Dissolve  in  water,  and  determine  the  aluminium  as 
follows. 

Procedure. — Dilute  the  solution  to  about  150  c.c.,  and  add 
$  c.c.  of  concentrated  hydrochloric  acid.  Warm  the  solution, 
and  add  a  moderate  excess  of  2N  ammonia  (30  to  35  c.c.). 
Pour  the  ammonia  down  the  stirring-rod  and  mix  it  with  the 


SULPHATE  AS  BARIUM  SULPHATE  131 

solution  by  stirring  at  intervals.  The  requisite  amount  of 
ammonium  chloride  is  produced  by  the  neutralisation  of 
the  hydrochloric  acid.  Precipitation  is  complete  when,  after 
rinsing  the  stirring-rod  and  the  side  of  the  beaker,  the  liquid 
is  found  to  smell  of  ammonia.  Heat  the  contents  of  the 
beaker  until  boiling,  and  boil  for  not  more  than  two  minutes. 
Filter  and  wash  the  precipitate  in  the  same  manner  as 
described  for  ferric  hydroxide  (p.  128). 

Dry  the  precipitate  (partially  at  least)  in  the  steam-oven. 
Incinerate  the  filter  in  presence  of  the  precipitate  in  a 
weighed  platinum  crucible  (p.  120).  Ignite  with  a  full  Bunsen 
flame  for  a  few  minutes  and  then  for  ten  minutes  with  a 
Meker  burner  or  a  blowpipe.  Cool,  and  weigh.  Repeat  the 
ignition  until  constant  weight  is  attained. 

Calculate  the  percentage  of  aluminium  in  the  ammonium 
aluminium  sulphate.  Record  all  weighings  and  state  the 
error  of  the  result  in  the  same  way  as  shown  on  p.  129. 

Determination  of  Sulphate  as  Barium  Sulphate. 

OUTLINE  OF  METHOD. — The  sulphate  is  precipitated  as  barium  sulphate 
by  the  addition  of  barium  chloride,  and  the  precipitate,  after 
ignition,  is  weighed  as  BaSO4. 

Barium  Sulphate,  obtained  by  precipitation,  is  a  fine  white 
powder  which  is  not  quite  insoluble  in  water.  At  18°,  i  litre 
of  water  dissolves  2-3  mgrms.  It  is  from  twenty  to  thirty 
times  more  soluble  in  cold  dilute  (normal)  hydrochloric  and 
nitric  acids.  It  dissolves  freely  in  concentrated  sulphuric 
acid,  but  is  reprecipitated  on  diluting  the  acid.  In  dilute 
sulphuric  acid  and  in  barium  chloride  solution  it  is  practically 
insoluble.  Barium  sulphate  may  be  ignited  in  air  at  a  red 
heat  without  alteration  of  weight. 

A  source  of  error  in  the  determination  of  sulphate  is  that 
occasioned  by  the  marked  tendency  of  barium  sulphate  to 
carry  down  traces  of  other  substances  contained  in  the 
solution.  In  some  cases,  the  co-precipitated  substances 
cannot  be  removed  by  washing  or  ignition,  and  the  results 
are  accordingly  high ;  in  other  cases,  loss  of  sulphuric  acid 
may  occur  on  ignition,  and  low  results  may  be  obtained.  In 
order  to  reduce  the  error  to  a  minimum,  and  to  obtain  a 


132  GRAVIMETRIC  ANALYSIS 

granular    precipitate    suitable    for    filtration,    the    following 
conditions  must  be  observed : — 

1.  The  solution  must  be  free  from  iron  (ferric),  aluminium, 

chromium,  nitrate  and  chlorate.  Iron  can  be  re- 
moved by  precipitation  with  ammonia;  nitrate  and 
chlorate  by  repeated  evaporation  with  concentrated 
hydrochloric  acid. 

2.  The  volume  of  the  solution  should  not   be  less  than 

250  c.c.  for  each  0-5  gram  of  barium  sulphate,  and 
should  contain  a  little  hydrochloric  acid  (about  I  per 
cent,  by  volume  of  the  dilute  acid). 

3.  The  barium  chloride  solution  should  be  dilute  (about 

3  per  cent.),  and  may  be  acidified  with  a  few  drops 
of  dilute  hydrochloric  acid. 

4.  Precipitation  must  take  place  slowly,  the  hot  barium 

chloride  solution  being  added  drop  by  drop  to  the 
nearly  boiling  sulphate  solution,  and  an  excess  (about 
2  c.c.)  of  the  barium  chloride  should  be  introduced 
after  the  precipitation  is  complete.1 

Exercise. — Weigh  accurately  about  0-6  gram  of  magnesium 
sulphate,  MgSO4,  7H2O.  Transfer  it  to  a  400  c.c.  beaker, 
dissolve  in  water,  and  determine  the  sulphate  as  follows. 

Procedure. — Dilute  the  solution  to  about  250  c.c.,  add  2  c.c. 
of  dilute  hydrochloric  acid,  and  heat  until  boiling.  Prepare  an 
approximately  3  per  cent,  solution  of  barium  chloride  (BaCl9, 
2H2O),  acidify  about  20  c.c.  of  the  solution  with  a  few  drops 
of  dilute  hydrochloric  acid,  and  heat  until  boiling.  Lower  the 
flame  under  the  magnesium  sulphate  solution  until  the  latter 
just  ceases  to  boil,  rinse  the  cover  glass  into  the  beaker,  and 
add  the  hot  barium  chloride  solution  drop  by  drop  (see  note 
below) ;  stir  constantly  while  precipitation  is  in  progress. 

When  the  precipitation  appears  to  be  complete,  allow  the 
precipitate  to  settle,  and  ascertain  whether  the  addition  of  a 
few  more  drops  of  barium  chloride  produces  any  further 
precipitate.  After  all  the  sulphate  is  precipitated,  add  an 

1  If  the  barium  chloride  is  added  rapidly,  the  precipitate  will  contain 
an  appreciable  amount  of  chloride.  After  precipitation  is  complete, 
however,  an  excess  of  barium  chloride  may  safely  be  added,  in  order  to 
diminish  the  solubility  of  the  barium  sulphate. 


CHLORIDE  AS  SILVER  CHLORIDE  133 

additional  2  c.c.  of  barium  chloride  solution,  stir  briskly,  and 
then  set  the  beaker  aside  for  about  an  hour. 

Decant  the  clear  liquid  through  a  9  cm.  filter,  and  wash 
the  precipitate  twice  with  hot  water  (by  decantation). 
Transfer  the  precipitate  to  the  filter.  Wash  the  precipitate 
and  the  filter  with  hot  water,  until  a  portion  of  the  filtrate 
gives  no  turbidity  with  a  few  drops  of  silver  nitrate. 

Incinerate  the  filter  in  a  weighed  crucible  in  the  manner 
described  on  p.  120.  After  all  the  carbon  is  burned,  allow 
the  crucible  to  cool.  In  order  to  convert  into  sulphate  any 
sulphide  that  may  have  been  formed  during  the  burning  of 
the  filter  paper,  add  2  or  3  drops  of  a  mixture  consisting  of 
i  c.c.  of  alcohol  and  2  drops  of  concentrated  sulphuric  acid. 
Warm  very  gently  until  the  excess  of  sulphuric  acid  has 
volatilised,  and  then  ignite  with  a  full  Bunsen  flame  for 
ten  minutes.  Cool,  and  weigh.  Repeat  the  ignition  until 
constant  weight  is  attained. 

From  the  weight  of  barium  sulphate  obtained,  calculate 
the  percentage  of  sulphate  (SO4)  in  the  magnesium  sulphate. 

Note. — A  simple  form  of  dropping  tube,  by  means  of 
which  the  barium  chloride  (or  other  reagent)  can  be  added 
slowly,  is  made  by  drawing  out  a  test-tube  in  the  blowpipe 
flame  so  as  to  form  a  capillary  through  which  the  solution 
will  pass  at  the  rate  of  about  2  drops  per  second.  The  tube, 
charged  with  the  hot  barium  chloride  solution,  is  supported 
over  the  beaker  in  a  clean  clamp. 

Determination  of  Chloride  as  Silver  Chloride. 

OUTLINE  OF  METHOD.— The  chloride  is  precipitated  as  silver  chloride 
by  the  addition  of  silver  nitrate.  The  precipitate  is  filtered  in  the 
usual  way,  and,  after  incinerating  the  filter,  is  weighed  as  AgCl ;  or, 
preferably,  the  precipitate  is  collected  and  weighed  in  a  Gooch 
crucible. 

Silver  Chloride  is  not  quite  insoluble  in  water.  At  18°, 
i  litre  of  water  dissolves  1-3  mgrm.  It  is  much  more 
soluble  in  hot  water,  i  litre  of  which,  at  100°,  dissolves  nearly 
22  mgrms.  ;  for  this  reason,  a  silver  chloride  precipitate 
must  be  washed  with  cold  water.  The  solubility  in  very 
dilute  hydrochloric  and  nitric  acids  and  in  dilute  silver 


134  GRAVIMETRIC  ANALYSIS 

nitrate  solution  is  negligibly  small ;  on  the  other  hand,  silver 
chloride  is  decidedly  soluble  in  concentrated  hydrochloric 
acid,  and  in  concentrated  solutions  of  silver  nitrate  and  most 
chlorides  (i  litre  of  saturated  sodium  chloride  dissolves  I  gram 
of  silver  chloride). 

On  exposure  to  sunlight,  silver  chloride  loses  chlorine? 
becoming  first  violet  and  then  nearly  black ;  and,  although 
this  change  is  at  first  superficial,  the  loss  of  weight  is  appreci- 
able. Silver  chloride  melts  at  about  490°  and  volatilises. 

Exercise. — Weigh  accurately  about  04  gram  of  barium 
chloride,  BaCl2,2H2O.  Transfer  it  to  a  300  c.c.  beaker, 
dissolve  in  water,  and  determine  the  chloride  as  follows. 

Procedure. — Dilute  the  solution  to  about  100  c.c.,  and  add 
5  c.c.  of  dilute  nitric  acid.  To  the  cold  solution  add  silver 
nitrate  solution  gradually,  whilst  stirring  briskly,  until  pre- 
cipitation of  the  chloride  is  complete.  A  large  excess  of 
silver  nitrate  must  not  be  added  and  is  easily  avoided, 
since  the  precipitate  coagulates  as  soon  as  a  small  excess 
of  silver  nitrate  is  present.  In  order  to  protect  the  silver 
chloride  from  bright  light,  wrap  a  piece  of  brown  paper 
round  the  beaker  (use  a  rubber  band  to  fix  the  paper  in 
place).  Place  the  beaker  on  the  steam-bath,  and  stir  the 
liquid  frequently  until  the  precipitate  has  completely  coagu- 
lated and  the  liquid  is  perfectly  clear.  Make  certain  that 
precipitation  is  complete  by  adding  another  drop  of  silver 
nitrate,  and  then  allow  the  solution  to  cool. 

Decant  the  clear  liquid  through  a  9  cm.  filter,  and  wash 
the  precipitate  several  times  by  decantation  with  cold  water 
containing  a  few  drops  of  nitric  acid.  Transfer  the  precipitate 
to  the  filter  in  the  usual  way,  and  wash  with  cold  water 
acidified  with  nitric  acid,  until  a  portion  of  the  filtrate  gives 
no  turbidity  with  dilute  hydrochloric  acid.  Finally,  wash 
with  pure  water  until  the  filtrate  is  free  from  acid  (test  with 
litmus  paper).  Dry  the  precipitate  in  the  steam-oven. 

Incinerate  the  filter  in  a  porcelain  crucible  apart  from  the 
precipitate,  in  the  manner  described  on  p.  121.  The  carbon 
should  be  burned  at  as  low  a  temperature  as  possible.  By 
means  of  a  glass  rod,  add  2  drops  of  concentrated  nitric 
acid  to  the  ash  in  the  crucible  and  warm  gently ;  then  add 


MAGNESIUM  AS  PYROPHOSPHATE  135 

one  drop  of  concentrated  hydrochloric  acid  and  cautiously 
evaporate  to  dryness.  (The  object  of  this  procedure  is  to 
convert  into  silver  chloride  the  metallic  silver  produced  during 
the  incineration  of  the  filter.)  Transfer  the  precipitate  to  the 
crucible,  and  either  heat  the  open  crucible  for  five  minutes 
with  a  very  small  flame,  great  care  being  taken  not  to  fuse 
the  precipitate,  or  dry  the  precipitate  in  the  air-oven  at  130° 
for  an  hour.  Cool,  and  weigh. 

A  more  convenient  method  of  filtering  silver  chloride  is  by 
means  of  a  Gooch  crucible.  The  asbestos  filter  is  prepared 
in  the  manner  described  on  p.  112,  and  the  crucible  is  dried 
in  the  air-oven  at  130°  and  weighed.  After  collecting  and 
washing  the  precipitate  in  the  crucible,  the  latter  is  again 
heated  for  an  hour  in  the  oven  at  the  same  temperature  as 
before,  and  is  then  cooled  and  weighed. 

From  the  weight  of  silver  chloride  obtained,  calculate  the 
percentage  of  chloride  in  the  barium  chloride. 

Determination  of  Magnesium  as  Pyrophosphate. 

(Precipitation  as  Magnesium  Ammonium  Phosphate?) 

OUTLINE  OF  METHOD. — The  magnesium  is  precipitated  as  magnesium 
ammonium  phosphate  by  means  of  sodium  ammonium  hydrogen 
phosphate  (microcosmic  salt).  The  precipitate  is  converted  into 
magnesium  pyrophosphate  by  ignition,  and  is  weighed  as  Mg2P2O7. 

Magnesium  Ammonium  Phosphate,  MgNH4PO4,6H2O,  is 
a  white  crystalline  substance,  which  is  somewhat  soluble  in 
water.  At  the  ordinary  temperature,  i  litre  of  water 
dissolves  about  65  mgrms.  It  is  much  less  soluble  in 
ammonia;  i  litre  of  $N  ammonia  dissolves  about  4  mgrms. 

In  order  to  obtain  a  precipitate  of  normal  composition 
(MgNH4PO4),  the  solution  must  be  neutral  and  as  free  as 
possible  from  ammonium  salts,  and  excess  of  phosphate  must 
be  avoided.  If  much  ammonia  or  ammonium  salt  is  present, 
the  precipitate  contains  Mg3(PO4)2  or  Mg(NH4)4(PO4)2; 
the  former  is  unchanged  by  ignition,  and  the  latter  gives 
magnesium  metaphosphate,  whereas  the  precipitate  of  normal 
composition,  MgNH4PO4,  is  converted  into  magnesium 
pyrophosphate. 


136  GRAVIMETRIC  ANALYSIS 

The  microcosmic  salt  precipitates  amorphous  magnesium 
hydrogen  phosphate, 

2MgSO4  +  2NaNH4HPO4  =  2MgHPO4  +  Na2SO4  +  (NH4)2SO4, 

and    ammonia    converts    this    into    crystalline    magnesium 
ammonium  phosphate, 

MgHPO4  +  NH3  =  MgNH4PO4. 

In  presence  of  much  ammonium  salt,  a  double  precipitation 
is  necessary;  the  procedure  is  described  on  p.  231. 

Magnesium  Pyrophosphate,  Mg2P2O7,  is  unchanged  by 
ignition  in  air,  but  if  reducing  gases  have  access,  phosphorus 
and  volatile  phosphorus  compounds  escape,  and  normal 
magnesium  orthophosphate  is  formed.  Magnesium  pyro- 
phosphate  fuses  at  a  high  temperature. 

Exercise. — Weigh  accurately  about  06  gram  of  magnesium 
sulphate,  MgSO4,7H2O.  Transfer  it  to  a  300  c.c.  beaker, 
dissolve  in  water,  and  determine  the  magnesium  as  follows. 

Procedure. — Dilute  the  solution  to  about  100  c.c.,  and 
heat  until  boiling.  Add  a  freshly  prepared  5  per  cent, 
solution  of  microcosmic  salt,  drop  by  drop,  until  no  more 
precipitate  forms.  A  large  excess  of  phosphate  must  be 
avoided.  Allow  the  solution  to  cool,  add  10  c.c.  of  concen- 
trated ammonia,  and  stir  briskly.  Cover  the  beaker  and  set 
it  aside  for  two  or  three  hours. 

Decant  the  clear  solution  through  a  filter ;  use  an  1 1  cm. 
paper  if  the  precipitate  is  bulky.  Wash  the  precipitate  twice 
by  decantation  with  2\  per  cent,  ammonia  (75  c.c.  of  2N 
ammonia  diluted  to  100  c.c.).  Transfer  the  precipitate  to  the 
filter,  and  wash  with  dilute  ammonia  until  a  portion  of  the 
filtrate  gives  no  turbidity  with  dilute  hydrochloric  acid  and 
barium  chloride.  Dry  the  precipitate  in  the  steam-oven. 

Incinerate  the  filter,  apart  from  the  precipitate,  in  a 
weighed  porcelain  crucible  in  the  manner  described  on  p.  121. 
Carbonise  the  paper  and  burn  the  carbon  at  as  low  a 
temperature  as  possible.  If  the  carbon  burns  with  difficulty, 
allow  the  crucible  to  cool,  and  moisten  the  contents  with  a 
few  drops  of  concentrated  nitric  acid.  Evaporate  the  acid 
carefully,  and  then  heat  more  strongly  until  the  ash  is 
perfectly  white.  Allow  the  crucible  to  cool  again,  and  add 


ZINC  AS  OXIDE  137 

the  precipitate.  Heat  the  crucible  gently  until  ammonia  is 
no  longer  evolved,  gradually  increase  the  temperature,  and 
finally  heat  for  ten  minutes  with  a  Meker  burner.  Cool,  and 
weigh.  Repeat  the  ignition  until  constant  weight  is  attained. 

The  precipitate  may  be  collected  and  weighed  in  a  Gooch 
crucible  (see  p.  112).  After  washing  the  precipitate  with 
dilute  ammonia  (six  to  eight  times  should  suffice),  the 
crucible,  containing  the  precipitate,  is  dried  in  the  steam- 
oven,  and  is  then  placed  in  a  larger  nickel  or  platinum 
crucible  and  ignited  as  above. 

From  the  weight  of  magnesium  pyrophosphate  obtained, 
calculate  the  percentage  of  magnesium  in  magnesium 
sulphate. 

Determination  of  Zinc  as  Oxide. 

OUTLINE  OF  METHOD. — The  zinc  is  precipitated  as  basic  carbonate  by 
means  of  sodium  carbonate.  The  filter  is  incinerated,  apart  from 
the  precipitate,  and  the  latter  is  converted  into  zinc  oxide  by  ignition, 
and  weighed  as  ZnO. 

Basic  Zinc  Carbonate,  the  composition  of  which  varies 
according  to  the  conditions  of  precipitation,  is  a  white  powder, 
very  slightly  soluble  in  water,  and  readily  soluble  in  acids, 
alkali  hydroxides,  and  ammonia.  It  is  slightly  soluble  in 
sodium  carbonate,  and  excess  of  the  reagent  must  therefore 
be  avoided.  If  the  zinc  solution  contains  much  sulphate, 
sodium  carbonate  always  precipitates  some  basic  sulphate, 
and  the  precipitate,  after  filtration,  must  be  dissolved  again 
and  reprecipitated ;  with  a  small  amount  of  sulphate  this  is 
unnecessary.  The  basic  carbonate  is  converted  into  zinc 
oxide  by  ignition. 

Zinc  Oxide  is  yellow  when  hot,  but  almost  white  when 
cold.  It  maybe  heated  to  bright  redness  without  volatilisa- 
tion ;  but  if  carbonaceous  matter,  such  as  traces  of  filter  paper, 
is  present,  partial  reduction  to  metallic  zinc  occurs,  and  the 
zinc  volatilises  readily. 

Exercise. — Weigh  accurately  about  0-8  gram  of  zinc  sul- 
phate, ZnSO4,7H2O.  Transfer  it  to  a  300  c.c.  porcelain 
beaker  or  casserole,  dissolve  in  water,  and  determine  the 
zinc  as  follows. 


138  GRAVIMETRIC  ANALYSIS 

Procedure. — Dilute  to  about  100  c.c.,and  to  the  cold  solu- 
tion add  sodium  carbonate  drop  by  drop  until  a  faint  turbidity 
appears ;  then  heat  until  boiling.  In  this  way,  the  greater  part 
of  the  zinc  is  precipitated  as  basic  carbonate  free  from  alkali 
carbonate.  Now  add  i  c.c.  of  phenolphthalein  and  more 
sodium  carbonate  until  the  solution  becomes  distinctly  pink. 
Boil  for  several  minutes. 

After  the  precipitate  has  settled,  decant  the  clear  liquid 
through  a  9  cm.  filter,  and  wash  the  precipitate  three  times 
with  hot  water  by  decantation.  Transfer  the  precipitate  to 
the  filter,  and  continue  the  washing  until  a  portion  of  the 
filtrate  gives  no  turbidity  with  hydrochloric  acid  and  barium 
chloride. 

Dry  the  precipitate  and  the  filter  in  the  steam-oven. 
Separate  the  precipitate  as  completely  as  possible  from  the 
filter,  without,  however,  rubbing  off  any  paper  fluff,  and  wrap 
up  the  paper  in  the  manner  described  on  p.  122.  In  order  to 
prevent  as  far  as  possible  the  reduction  of  any  zinc  oxide 
still  adhering  to  the  filter  paper,  moisten  the  paper  with  a  few 
drops  of  ammonium  nitrate  solution,  and  dry  it  in  the  steam- 
oven  for  a  few  minutes.  Incinerate  the  paper  in  a  weighed 
porcelain  crucible  at  as  low  a  temperature  as  possible.  When 
all  the  carbon  is  burned,  add  the  precipitate,  and  heat  the 
crucible,  gently  at  first,  and  then  to  bright  redness  for  ten 
minutes.  Use  a  good  oxidising  flame  and  take  care  to 
exclude  flame  gases  during  the  ignition,  otherwise  reduction 
of  the  oxide  and  loss  of  zinc  (by  volatilisation)  will  occur. 
Cool,  and  weigh.  Repeat  the  ignition  until  constant  weight 
is  attained. 

From  the  weight  of  zinc  oxide  obtained,  calculate  the 
percentage  of  zinc  in  zinc  sulphate. 

Note. — In  order  to  avoid  the  risk  of  loss  during  the 
incineration  of  the  filter,  the  zinc  carbonate  may  be  filtered 
by  means  of  a  Gooch  crucible  (see  p.  112).  The  crucible, 
containing  the  precipitate,  is  dried  in  the  steam-oven  or 
air-oven,  and  is  then  placed  in  a  larger  porcelain  or  nickel 
crucible  and  ignited  with  a  full  Bunsen  flame. 


COPPER  AS  OXIDE  139 


Determination  of  Copper  as  Cupric  Oxide. 

OUTLINE  OF  METHOD. — The  copper  is  precipitated  as  hydrated  copper 
oxide  by  means  of  sodium  hydroxide.  The  filter  is  incinerated 
apart  from  the  precipitate,  and  the  latter  is  converted  into  cupric 
oxide  by  ignition,  and  is  weighed  as  CuO. 

Copper  Hydroxide,  precipitated  from  a  cold  solution,  is  a 
light  blue  substance  which  becomes  dark  brown  or  black 
when  boiled  with  the  alkaline  solution.  The  change  in 
colour  is  due  to  loss  of  water,  the  composition  of  the  black 
precipitate  being  probably  3CuO,H2O.  The  precipitate  is 
slightly  soluble  in  sodium  hydroxide  solution,  and  readily 
soluble  in  ammonia  and  in  dilute  acids.  Precipitation  is 
incomplete  in  presence  of  organic  matter  or  ammonium 
salts. 

Cupric  Oxide,  produced  from  the  hydrated  oxide  by 
ignition,  is  a  black,  hygroscopic  powder  which  remains 
unaltered  at  a  red  heat,  provided  reducing  gases  are  carefully 
excluded. 

Exercise. — Weigh  accurately  about  0-8  gram  of  copper 
sulphate,  CuSO4,  5H2O.  Transfer  it  to  a  400  c.c.  porcelain 
beaker  or  a  large  casserole,  dissolve  in  water,  and  determine 
the  copper  as  follows. 

Procedure. — Dilute  the  solution  to  about  150  c.c.  and 
heat  until  almost  boiling.  Remove  the  flame  and  add,  drop 
by  drop  whilst  stirring,  a  dilute  solution  of  sodium  hydroxide 
(see  below),  until  the  precipitate  becomes  permanently  dark 
brown  or  black.  A  large  excess  of  alkali  must  be  carefully 
avoided.  Boil  the  contents  of  the  covered  vessel  for  about 
one  minute,  and  then  allow  the  precipitate  to  subside.  Make 
certain  that  the  clear  liquid  is  alkaline  by  placing  a  drop  on 
red  litmus  paper,  afterwards  rinsing  the  litmus  paper  into  the 
vessel. 

Decant  the  clear  liquid  through  a  9  cm.  filter,  and  wash 
the  precipitate  several  times  with  hot  water  by  decantation. 
Transfer  the  precipitate  to  the  filter.  Wash  the  precipitate 
and  the  filter — especially  the  margin  of  the  latter — until  a 
portion  of  the  filtrate  gives  no  turbidity  on  adding  a  few 
drops  of  dilute  hydrochloric  acid  and  barium  chloride. 


140  GRAVIMETRIC  ANALYSIS 

It  frequently  happens  that  a  small  quantity  of  the  copper 
oxide  adheres  to  the  side  of  the  beaker  and  cannot  be 
detached  by  rubbing  with  a  feather.  In  order  to  remove  it, 
add  2  drops  of  dilute  nitric  acid,  and  bring  the  acid  into 
contact  with  the  entire  surface  of  the  beaker  by  means  of  the 
stirring-rod  ;  rinse  down  the  interior  of  the  beaker  with  a 
very  little  hot  water,  heat  the  solution  to  the  boiling  point 
over  a  minute  flame,  and  reprecipitate  the  copper  oxide  by 
adding  a  few  drops  of  sodium  hydroxide  (avoid  excess). 
Transfer  the  minute  precipitate  at  once  to  a  separate  small 
filter,  and  wash  thoroughly. 

Dry  both  filters  and  the  precipitate  very  thoroughly  in 
the  steam  -  oven.  Incinerate  the  filters,  apart  from  the 
precipitate,  in  a  weighed  porcelain  crucible  in  the  manner 
described  on  p.  121.  When  all  the  carbon  is  burned,  allow 
the  crucible  to  cool,  and  moisten  the  ash  with  2  drops  of 
concentrated  nitric  acid,  in  order  to  oxidise  any  reduced 
oxide  formed  during  the  incineration.  Heat  the  crucible 
very  gently  with  a  minute  flame  until  fuming  ceases,  and 
then  heat  to  dull  redness  for  about  a  minute.  When  the 
crucible  has  become  nearly  cold  again,  place  it  on  glazed 
paper,  and  carefully  transfer  the  main  precipitate  to  the 
crucible. 

Heat  the  copper  oxide  in  the  open  crucible  to  dull 
redness  for  five  minutes,  cool  in  a  desiccator  as  usual,  and 
weigh.  Repeat  the  ignition  until  constant  weight  is  attained. 
During  the  ignition  every  care  must  be  taken  that  reducing 
gases  are  excluded  from  the  interior  of  the  crucible,  and  for 
this  purpose  a  perforated  silica  plate,  instead  of  a  pipe-clay 
triangle,  may  be  used  to  support  the  crucible. 

From  the  weight  of  the  copper  oxide  obtained,  calculate 
the  percentage  of  copper  in  the  copper  sulphate. 

Note. — Pure  sodium  hydroxide,  prepared  from  metallic 
sodium,  must  be  used  for  the  precipitation  of  copper  oxide. 
In  order  to  obtain  it,  cut  about  0-5  gram  of  clean  sodium 
into  small  pieces,  and  drop  the  pieces,  one  by  one,  into  about 
30  c.c.  of  water  contained  in  a  porcelain  basin.  Commercial 
sodium  hydroxide  ("  purified  by  alcohol ")  must  not  be  used, 
as  it  contains  traces  of  organic  matter. 


COPPER  AS  CUPROUS  SULPHIDE  141 

Determination  of  Copper  as  Cuprous  Sulphide. 

OUTLINE  OF  METHOD. — The  copper  is  precipitated  as  cupric  sulphide 
by  means  of  hydrogen  sulphide.  The  filter  is  incinerated  apart 
from  the  precipitate,  and  the  latter  is  converted  into  cuprous  sulphide 
by  heating  in  a  current  of  hydrogen,  and  is  weighed  as  Cu2S. 

Cupric  Sulphide  is  a  black  precipitate,  practically 
insoluble  in  water,  in  hydrochloric  acid  (sN),  and  in 
sulphuric  acid.  It  dissolves  readily  in  nitric  acid,  with 
separation  of  sulphur.  Exposed  to  the  air  in  a  moist  state, 
it  oxidises  rapidly,  acquires  a  greenish  colour,  and  becomes 
soluble  in  water.  In  order  to  prevent  oxidation,  the  pre- 
cipitate must  be  washed  with  water  containing  hydrogen 
sulphide.  Heated  in  a  current  of  hydrogen,  cupric  sulphide 
is  converted  into  cuprous  sulphide  which,  if  air  is  excluded, 
may  be  ignited  at  a  high  temperature  without  decomposition. 

Exercise. — Weigh  accurately  about  0-8  gram  of  copper 
sulphate.  Transfer  it  to  a  300  c.c.  conical  flask,  dissolve  in 
water,  and  determine  the  copper  as  follows. 

Procedure. — Dilute  the  solution  (which  must  not  contain 
nitric  acid  or  nitrates)  to  about  1 50  c.c.,  add  3  c.c.  of  con- 
centrated sulphuric  acid,  and  heat  the  solution 
nearly  to  the  boiling  point.  Pass  a  slow  current 
of  hydrogen  sulphide  through  the  hot  solution 
until  the  precipitate  is  quite  black  and  settles 
quickly  and  the  supernatant  liquid  is  clear  and 
colourless.  The  rate  at  which  hydrogen  sul- 
phide is  absorbed  is  greatly  increased  if  the 
glass  inlet  tube  is  expanded  into  a  bulb  (Fig. 
42)  so  that  the  mouth  of  the  flask  is  almost  com- 
pletely closed.  The  precipitation  requires  at  least  half  an  hour. 

Meanwhile,  prepare  some  hydrogen  sulphide  solution  by 
passing  the  gas  into  water  contained  in  a  special  wash- 
bottle  fitted  with  a  valve  (p.  26). 

When  precipitation  is  complete,  remove  the  gas  delivery 
tube  and  rinse  it  into  the  flask ;  rub  off  any  adhering 
precipitate  by  means  of  a  prepared  feather,  and  rinse  the 
tube  again,  internally  as  well  as  externally,  and  also  the 
feather. 


142  GRAVIMETRIC  ANALYSIS 

Decant  the  clear  liquid  through  a  9  cm.  filter,  and,  with 
the  help  of  the  hydrogen  sulphide  solution,  transfer  the 
precipitate  at  once  to  the  filter.  Wash  the  precipitate  and 
the  filter,  especially  the  margin  of  the  latter,  with  hydrogen 
sulphide  solution,  until  a  portion  of  the  filtrate  is  found  to 
be  free  from  acid  when  tested  with  a  drop  of  methyl  orange. 
During  the  whole  process  of  filtration  and  washing,  the 
precipitate  must  be  kept  covered  with  the  washing  liquid 
as  far  as  possible ;  if  this  is  not  attended  to,  partial  oxidation 
and  solution  of  the  precipitate  will  occur,  and  the  filtrate 
will  become  turbid  and  acquire  a  greenish  colour. 

Dry  the  precipitate  in  the  steam-oven.  Incinerate  the 
filter  apart  from  the  precipitate  in  a  weighed  Rose  crucible 
(p.  121).  Having  made  sure  that  no  carbon  remains 
unburned,  allow  the  crucible  to  cool,  introduce  the  main 
precipitate,  and  also  a  little  (about  o-oi  gram)  finely  powdered, 
pure  sulphur.  Pass  a  current  of  pure  dry  hydrogen  (see 
p.  115)  into  the  crucible  at  the  rate  of  about  four  bubbles  per 
second,  and  heat  the  crucible,  gently  at  first,  and  then  with 
a  Meker  burner  for  ten  minutes.  The  excess  of  sulphur 
volatilises,  and  the  cupric  sulphide  is  converted  into  cuprous 
sulphide.  Remove  the  flame  and,  at  the  same  time, 
increase  the  rate  of  the  gas  current  somewhat,  and  allow  the 
crucible  to  cool.  (Make  sure  that  the  hydrogen  no  longer 
burns  at  the  crucible  by  pinching  the  rubber  connection  for 
an  instant.)  When  the  crucible  is  almost  cold,  transfer  it  to 
a  desiccator  for  ten  minutes,  and  then  weigh  it.  Repeat  the 
ignition  in  the  same  way  until  constant  weight  is  attained. 

The  cuprous  sulphide  must  appear  blue-black  or  black ; 
if  any  red-brown  particles  are  visible  (metallic  copper  or 
cuprous  oxide)  the  current  of  hydrogen  during  cooling  was 
too  slow,  and  the  ignition  with  sulphur  must  be  repeated. 

From  the  weight  of  cuprous  sulphide  obtained,  calculate 
the  percentage  of  copper  in  the  copper  sulphate. 


CALCIUM  AS  OXALATE  143 

Determination  of  Calcium  as  Oxalate. 

OUTLINE  OF  METHOD. — The  calcium  is  precipitated  as  calcium  oxalate 
by  means  of  ammonium  oxalate,  and  the  precipitate,  after  conversion 
into  calcium  carbonate  or  calcium  oxide,  is  weighed  as  CaCO3  or  CaO. 

Calcium  Oxalate^  CaC2O4,H2O,  is  a  fine  white  powder, 
which  is  very  slightly  soluble  in  water.  At  18°,  I  litre  of 
water  dissolves  about  5-5  mgrms.  In  dilute  ammonia  it  is 
somewhat  less  soluble  than  in  water.  It  dissolves  easily  in 
hydrochloric  and  nitric  acids,  but  very  sparingly  in  acetic 
acid.  It  is  somewhat  soluble  in  magnesium  chloride  solution. 
Dried  at  100°,  the  composition  of  the  precipitate  corresponds 
with  the  monohydrate ;  if  heated  to  a  temperature  approach- 
ing dull  redness,  it  is  converted  into  calcium  carbonate. 

Calcium  Carbonate  may  be  heated  to  about  500°  with- 
out appreciable  decomposition.  The  dissociation  pressure 
increases  (slowly  at  first  and  then  rapidly)  with  the  tempera- 
ture, and  becomes  equal  to  the  atmospheric  pressure  at  about 
812°.  If  heated  above  800°  in  a  vessel  from  which  the 
carbon  dioxide  can  escape,  it  is  completely  converted  into 
calcium  oxide. 

Calcium  Oxide  is  a  hygroscopic  substance,  and  should  be 
exposed  to  the  air  as  little  as  possible  during  weighing.  As 
it  also  absorbs  carbon  dioxide  readily,  it  should  be  kept 
in  a  desiccator  containing  soda-lime  or  sticks  of  sodium 
hydroxide. 

Exercise. — Weigh  accurately  0-4  to  0-5  gram  of  powdered 
calcite  (calcspar).  Transfer  it  to  a  400  c.c.  beaker.  Add 
about  10  c.c.  of  water,  cover  the  beaker  with  a  clock-glass, 
and  dissolve  the  calcite  by  adding  dilute  hydrochloric  acid 
(about  10  c.c.).  Dilute  with  a.  little  water,  and  boil  the 
solution  for  a  few  minutes  in  order  to  free  it  from  carbon 
dioxide.  Determine  the  calcium  as  follows. 

Procedure. — Add  a  few  drops  of  methyl  orange,  and 
exactly  neutralise  the  solution  with  ammonia.  Then  add 
i  c.c.  of  dilute  hydrochloric  acid,  dilute  the  solution  to  about 
200  c.c.,  heat  until  boiling,  and  add  a  moderate  excess  of  a 
boiling  solution  of  ammonium  oxalate  (freshly  prepared  cold 
saturated  solution).  Then  make  the  mixture  alkaline  with 


144  GRAVIMETRIC  ANALYSIS 

ammonia,  and  boil  for  a  few  minutes.     Set  the  beaker  aside 
for  one  hour. 

Filter  through  a  9  cm.  paper,  and  wash  the  precipitate 
three  times  by  decantation  with  warm  water  containing  a 
little  ammonia.  Transfer  the  precipitate  to  the  filter,  and 
continue  the  washing  until  a  portion  of  the  filtrate  gives  no 
turbidity  with  nitric  acid  and  silver  nitrate.  (Too  prolonged 
washing  must  be  avoided,  on  account  of  the  decided 
solubility  of  calcium  oxalate.) 

(1)  If  the  precipitate  is  to  be  converted  into  and  weighed  as 
calcium   oxide,  incinerate   the   filter,  together   with   the   still 
moist   precipitate,  in   a  weighed   platinum   crucible,  in   the 
manner  described  on  p.  120.     After  all  the  carbon  is  burned, 
heat   the   crucible,  gently  at   first,  and   then  with  a  Meker 
burner   for  twenty  minutes.     Cool,  and  weigh.     Repeat  the 
ignition  (for  ten  minutes)  until  constant  weight  is  attained. 

(2)  If  the  precipitate  is  to  be  converted  into  and  weighed  as 
calcium  carbonate,  either  a  porcelain  or  a  platinum  crucible 
may   be    used.      The    procedure   is    as    follows: — Dry   the 
precipitate  in   the  steam-oven.     Incinerate   the   filter,  apart 
from   the   precipitate,  in   the   manner  described  on  p.  121. 
Moisten     the     ash     with     2     drops     of     freshly     prepared 
ammonium  carbonate  solution,  and  evaporate  very  carefully 
to  dryness.     Transfer  the  precipitate  to  the  crucible  and  heat 
the  latter  gently  at   first,  and   then   for  ten   minutes  more 
strongly  until  the  bottom  of  the  crucible  reaches  very  faint 
redness  (when  shaded  from  direct  light).     An  Argand  burner, 
provided  with  an  iron  chimney,  is  very  convenient  for  this 
purpose,  and  for  similar  ignitions  requiring  a  temperature  not 
exceeding   dull   redness ;   the  pipe-clay  triangle  supporting 
the  crucible  is  placed  on  the  top  of  the  chimney.     Cool,  and 
weigh.      In   order   to   convert   into   carbonate   any   calcium 
oxide  that  may  have  been  formed  by  overheating,  moisten 
the  precipitate  with  a  few  drops  of  ammonium   carbonate 
solution,  dry  in  the  steam-oven,  and  then  heat  gently  with  a 
very  small  flame  until  the  excess  of  ammonium  carbonate  has 
volatilised.     Cool,  and  weigh.     Repeat   the   treatment  with 
ammonium  carbonate  until  constant  weight  is  attained. 

From  the  weight  of  calcium  oxide  or  calcium  carbonate 
obtained,  calculate  the  percentage  of  calcium  in  calcite. 


Electrolytic  Methods 


When  a  current  of  electricity  is  passed  through  a  solution 
of  a  metallic  salt,  the  salt  is  decomposed  and  the  products  of 
the  electrolysis  appear  at  the  electrodes.  With  a  simple  salt 
solution,  metal  is  deposited  on  the  cathode  unless  the  metal 
present  in  the  salt  is  one  which  decomposes  water. 

Theoretically,  all  metals  not  attacked  by  water  may  be 
precipitated  from  solutions  of  their  salts  by  electrolysis,  but 
it  is  necessary  for  gravimetric  purposes  that  the  deposit 
should  be  pure.  In  some  cases,  it  is  a  matter  of  great 
difficulty  to  obtain  such  a  pure  deposit ;  in  other  cases, 
electrolytic  methods  have  been  found  to  yield  results  of  a 
high  degree  of  accuracy.  Electrolytic  methods  are  confined 
almost  entirely  to  the  determination  of  metallic  radicals  :  and, 
as  a  rule,  the  metal  is  deposited  as  such;  in  the  case  of  lead, 
it  is  found  better  so  to  adjust  the  conditions  of  electrolysis 
that  the  lead  is  deposited  as  lead  dioxide  on  the  anode.  If 
a  metal  is  to  be  determined  electrolytically,  it  must  (i)  be 
deposited  in  a  pure  state ;  (2)  be  deposited  completely  from 
the  solution ;  and  (3)  form  a  coherent  deposit  on  the 
electrode. 

The  nature  of  the  deposit  depends  on  a  number  of 
conditions,  among  which  may  be  mentioned  the  rate  of 
deposition,  the  composition  of  the  solution,  and  the  tempera- 
ture. The  correct  conditions  vary  for  different  metals,  and, 
in  most  cases,  a  method  becomes  inaccurate  if  all  the 
conditions  are  not  adjusted  within  fairly  narrow  limits;  this 
is  one  of  the  main  objections  to  electrolytic  methods,  since 
it  is  not  always  easy  or  even  possible  to  conform  to  these 
conditions.  For  example,  copper  is  readily  deposited  using 
a  2-volt  current,  but  in  presence  of  a  large  quantity  of  iron 
the  copper  is  no  longer  completely  precipitated. 

145  K 


146 


GRAVIMETRIC  ANALYSIS 


Separations. — In  most  cases,  metals  can  be  quantitatively 
separated  from  one  another  by  electrolytic  methods.  There 
are  two  main  ways  by  which  the  separation  may  be  effected : 
(i)  by  suitable  adjustment  of  the  voltage  ;  and  (2)  by  suitable 
alteration  of  the  composition  of  the  solution.  In  order  to 
illustrate  this,  two  methods  used  to  separate  copper  from 
nickel  may  be  mentioned.  The  copper  is  deposited  from  a 
solution  of  the  mixed  sulphates  by  electrolysis  with  a  2-volt 
current  (with  this  voltage,  no  nickel  is  deposited) ;  after  the 
copper  has  been  removed,  the  nickel  may  be  deposited  by 
using  a  higher  voltage.  The  same  separation  may  be 
effected  with  a  fixed  voltage  (say  4  volts).  The  copper 
is  first  deposited  in  presence  of  nitric  acid ;  when  the 
copper  has  been  removed,  the  nickel  is  deposited  by 
further  electrolysis  after  adding  an  excess  of  ammonia  to 
the  solution. 


*j$    Battery 


Ammeter 
FlG.  43. — General  Arrangement  of  the  Apparatus. 

Composition  of  the  Solution. — From  the  above  illustra- 
tion it  is  evident  that  this  is  of  importance.  In  certain  cases 
it  is  necessary  to  add  an  oxalate  or  tartrate  to  the  solution, 
as  otherwise  it  is  impossible  to  deposit  the  whole  of  the 
metal. 

The  degree  of  acidity  is  often  important.  For  obvious 
reasons,  hydrochloric  acid  must  be  absent  from  a  solution 
to  be  electrolysed. 


D 


ELECTROLYTIC  METHODS  147 

Source  of  Current. — By  far  the  most  satisfactory  source 
of  current  for  this  work  is  a  battery  of  lead  accumulators, 
capable  of  giving  up  to  5  amperes  at  an  E.M.F.  of  2  to  10 
volts.  The  voltage  of  the  accumulators  must  be  tested  from 
time  to  time.  It  is  not  advisable  to  run  an  accumulator 
after  the  voltage  has  fallen  below  1-9  volts,  and  it  must  never 
be  used  after  the  voltage  has  fallen  to  1-8  volts.  Batteries  of 
Bunsen  or  Daniell  cells  may  also  be  used,  but  are  less 
convenient  than  accumulators. 

Electrodes. — For  most  purposes,  a  platinum  basin  holding 
about  150  c.c.  of  liquid  is  the  most 
convenient  cathode.  The  inner 
surface  should  be  roughened  (suit- 
able basins  with  the  surface 
roughened  by  a  sand-blast  may 
be  purchased)  in  order  that  the 
deposit  may  adhere  firmly.  This 
roughened  surface  must  never  be 

cleaned  with  sand  or  other  abra-     <^~~\ — -x. 

KU_^ 


sive  material. 

As  anode,  a    stout   perforated  FlG 

platinum  disc,  shaped  like  a  saucer 

(Fig.  44,  A),  may  be  used,  or  a  stout  platinum  wire  may 
be  wound  in  a  flat  spiral  as  shown  in  B  (Fig.  44).  The 
stout  platinum  wire  D  is  used  to  clamp  the  electrode  in 
position  and  to  make  the  electrical  connection. 

Stand. — A  stand  with  insulation  between  the  positive  and 
negative  terminals  is  convenient ;  but  if  a  special  stand  is  not 
available,  the  necessary  insulation  may  be  obtained  by  clamp- 
ing the  anode  support  D  in  a  rubber  cork. 

Siphon. — When  the  deposition  is  complete,  it  is  often 
necessary  to  remove  the  electrolyte  without  stopping  the 
current,  as  otherwise  partial  re-solution  of  the  deposit  would 
occur.  This  is  most  easily  accomplished  by  means  of  a 
siphon  arranged  as  shown  in  Fig.  45.  The  short  limb  of 
the  siphon  should  reach  to  the  bottom  of  the  basin,  and  to 
prevent  abrasion  it  should  be  covered  with  a  short  piece  of 
rubber  tubing.  The  longer  limb  should  be  fitted  with  a 
rubber  tube  and  screw-clip,  by  means  of  which  the  rate  of 


148 


GRAVIMETRIC  ANALYSIS 


outflow  may  be  regulated.  To  start  the  siphon,  fill  it  with 
water,  place  it  in  position,  and  open  the  screw-clip.  Run 
water  into  the  basin  from  a  tap-funnel 
to  replace  the  water  withdrawn.  The 
water  must  be  allowed  to  flow  gently 
on  to  the  surface  of  the  solution  so  that 
there  is  as  little  mixing  as  possible. 
When  about  200  c.c.  of  water  has  been 
used,  stop  the  current,  disconnect  the 
apparatus,  and  complete  the  washing  in 
the  usual  manner  with  the  wash-bottle. 

Measurement  of  Current. — An  am- 
meter reading  up  to  5  or  6  amperes 
(not  necessarily  very  accurately)  and  a 
voltmeter  recording  up  to  5  volts  are 
required. 

Regulation  of  Current. — It  is  advis- 
able to  have  some  form  of  adjustable 
resistance,  such  as  a  rheostat  with  a 

sliding  contact,  in  the  circuit.  A  rheostat  with  a  maximum 
resistance  of  10  ohms  will  be  found  suitable  for  most 
purposes. 


Electrolytic  Determination  of  Copper. 

(  With  Stationary  Electrodes^) 

Copper  is  readily  deposited  from  a  copper  sulphate 
solution  by  electrolysis,  and  forms  a  coherent  deposit  if  the 
potential  used  is  not  above  2-2  volts.  Further,  the  copper 
under  these  conditions  is  deposited  in  a  pure  state,  even 
when  the  solution  contains  iron,  nickel,  and  other  metals. 
The  time  required  for  complete  deposition  is  greatly 
increased  when  iron  is  present  in  the  solution,  but  the 
precipitation  is  quantitative,  even  in  presence  of  I  gram  of 
iron,  if  sufficient  time  is  allowed.  Nitrate  and  chloride 
interfere  with  this  method,  and  if  these  radicals  are  pre- 
sent the  solution  must  be  evaporated  with  concentrated 
sulphuric  acid  until  complete  conversion  into  sulphate  is 
effected. 


ELECTROLYTIC  METHODS  149 

Procedure. — Clean  the  platinum  basin  with  sodium 
hydroxide  in  order  to  remove  grease,  then  with  nitric 
acid,  and  finally  with  water.  Drain  the  dish — but  do  not 
touch  the  interior  with  the  fingers  or  wipe  it  with  a 
cloth — place  it  in  the  steam-oven  for  half  an  hour,  cool, 
and  weigh. 

Dilute  the  solution  to  100  c.c.,  and,  if  there  is  not  some  free 
sulphuric  acid  already  present,  add  5  c.c.  of  dilute  sulphuric 
acid.  Place  the  solution  in  the  tared  platinum  dish,  and  use 
an  anode  of  sheet  platinum  or  stout  platinum  wire.  Connect 
the  electrodes  with  an  accumulator,  without  any  intermediate 
resistance,  and  allow  the  current  to  pass  for  about  twelve 
hours.  It  is  often  convenient  to  start  the  experiment  late  in 
the  day  and  allow  the  current  to  pass  all  night. 

When  the  deposition  is  apparently  complete,  pour  or 
siphon  the  liquid  quickly  into  a  beaker,  immediately  after 
the  current  is  broken.  Wash  with  a  little  water  and  then 
twice  with  alcohol,  dry  for  not  more  than  five  minutes  in  the 
steam-oven,  cool,  and  weigh. 

Pour  back  the  solution  and  pass  the  current  again  for 
an  hour,  and  re-weigh.  If  the  weight  is  unchanged,  it 
may  be  assumed  that  all  the  copper  is  deposited.  If,  how- 
ever, large  quantities  of  other  metals  (particularly  iron)  are 
present,  the  solution  may  still  contain  copper. 

The  results  obtained  by  this  method  are  consistently  low 
by  about  i  mgrm.  This  last  trace  of  copper  may  be  removed 
by  increasing  the  potential  to  4  volts  for  the  last  half  hour, 
but  this  is  not  always  permissible. 

Electrolytic  Determination  of  Cadmium. 

Cadmium  is  readily  deposited  electrolytically  from 
solutions  of  most  cadmium  salts,  but  in  order  to  obtain 
a  pure  coherent  deposit  it  is  best  to  use  a  solution  of 
potassium  cadmium  cyanide.  Since  the  cadmium  in  this 
salt  forms  part  of  the  complex  acidic  radical,  it  is  deposited 
on  the  anode. 

Procedure. — To  the  cadmium  solution  add  I  c.c.  of  phenol- 
phthalein  solution,  and  then  add  pure  sodium  hydroxide 
solution  until  a  permanent  pink  coloration  is  produced. 


150  GRAVIMETRIC  ANALYSIS 

Prepare  a  dilute  solution  (about  5  per  cent.)  of  potassium 
cyanide  and  add  this,  with  constant  stirring,  until  the  pre- 
cipitate has  just  redissolved.  Carefully  avoid  adding  any 
excess  of  potassium  cyanide  beyond  that  necessary  to 
completely  dissolve  the  precipitate. 

Dilute  to  about  120  c.c.  and  electrolyse  in  a  tared 
platinum  basin,  arranged  as  for  the  determination  of  copper, 
except  that  the  basin  must  be  made  the  anode.  Use  a 
platinum  cathode.  Connect  the  electrodes  with  the  terminals 
of  a  6-  to  8- volt  battery,  and,  by  means  of  a  rheostat,  adjust 
the  current  so  that  only  about  0-5  ampere  passes  at  first. 
The  electrode  potential  difference  should  be  about  5  volts. 
After  five  to  six  hours'  electrolysis  with  this  current,  in- 
crease the  current  to  i-o — 1-2  amperes,  and  continue  the 
electrolysis  for  another  hour. 

After  stopping  the  current,  pour  off  the  liquid  at  once,  and 
rinse  the  basin  immediately  with  water ;  then  rinse  with 
alcohol,  and  finally  with  ether.  Dry  for  a  few  minutes  in 
the  steam-oven,  cool  in  a  desiccator,  and  weigh. 

Electrolytic  Determination  of  Copper. 

(With  a  Rotating  Cathode?) 

If  the  deposit  of  copper  is  to  be  adherent,  it  is  essential 
that  there  should  be  no  simultaneous  liberation  of  hydrogen 
at  the  cathode.  The  evolution  of  hydrogen  may  be  prevented 
in  two  ways  : — (i)  By  use  of  a  current  not  exceeding  2-2  volts, 
as  the  evolution  of  hydrogen  at  voltages  below  this  is  negligibly 
small.  (Copper,  however,  cannot  be  deposited  quantitatively 
from  a  solution  containing  nitric  acid  by  a  2-2-volt  current.) 
(2)  By  electrolysis  in  presence  of  nitric  acid,  the  nitric  acid 
acting  as  a  depolariser.  This  method  is  more  generally 
useful  than  the  first  method,  since  nitric  acid  is  the  acid 
usually  employed  to  dissolve  copper  alloys. 

The  time  necessary  for  complete  deposition  is  always 
greatly  shortened  by  using  a  rotating  electrode;  and  this 
device  is  particularly  useful  when  copper  is  to  be  deposited 
from  a  solution  containing  nitric  acid,  since  it  prevents  local 
accumulation  of  nitrous  acid  at  the  cathode. 

The  following  method  for  the  determination  of  copper  is 


ELECTROLYTIC  METHODS 


151 


convenient  and  accurate,  and  it  may  be  used  to  separate 
copper  from  tin,  nickel,  zinc,  and  iron.  The  solution  must 
contain  from  5  to  10  per  cent,  of  nitric  acid.  It  must  be  free 
from  chloride  and  nitrite,  both  of  which  may  be  removed,  if 
necessary,  by  evaporation  with  sulphuric  acid. 

The  main  difficulty  in  this  method  is  to  prevent  re- 
solution of  copper  during  the  washing  process  ;  this  is  due  to 
the  presence  of  nitrous  acid,  formed  by  electrolytic  reduction 
of  the  nitric  acid.  If  the  nitrous  acid  is  destroyed  by  adding 
a  little  hydrogen  peroxide  or  urea,  no  copper  is  dissolved 
by  the  nitric  acid. 

Procedure. — The  amount  of  material  taken  should  prefer- 
ably be  such  as  will  yield  about  0-3  gram  of  copper.  Dilute 
to  100  c.c.  and  add  sufficient  nitric  acid  to  bring  its  con- 
centration up  to  5 — 7  per  cent. 


FlG.  46. — General  Arrangement  of  the  Apparatus  with  a  Rotating  Electrode. 


The  electrolysis  is  best  conducted  in  a  deep,  narrow 
beaker.  The  copper  is  deposited  on  a  tared,  stout,  nickel 
wire  arranged  in  a  spiral  as  shown  in  Fig.  46.  This  nickel 


152  GRAVIMETRIC  ANALYSIS 

cathode  is  connected  with  the  negative  pole  of  a  4-volt 
battery  (two  accumulator  cells  in  series)  by  means  of  a  brass 
spring  which  presses  on  the  upper,  projecting  end  of  the 
wire.  A  resistance  should  be  placed  in  the  circuit  so  that 
the  current  may  be  readily  controlled,  and  an  ammeter  to 
measure  the  current  is  desirable,  but  not  indispensable.  Use 
a  sheet  of  stout  platinum  foil  (about  6  square  inches)  as  anode. 
The  platinum  sheet  may  be  made  much  more  rigid,  if  neces- 
sary, by  making  parallel  corrugations  in  it  by  means  of  a 
blunt  instrument,  such  as  a  spatula.  Fasten  the  anode  so 
that  there  is  no  chance  of  it  touching  the  rotating  cathode ; 
this  may  be  done  conveniently  by  means  of  two  or  three 
D-shaped  pieces  of  glass  rod  which  hold  the  anode  flat 
against  the  side  of  the  beaker.  The  nickel  electrode  is  rotated 
at  a  high  speed  by  means  of  a  small  electric  motor.1 

Before  closing  the  circuit^  start  the  cathode  rotating  in  order 
to  make  certain  that  the  electrodes  do  not  touch. 

Pass  a  current  of  about  0-5  ampere  at  first,  and,, after  a 
few  minutes,  increase  it  until  from  2  to  10  amperes  are 
passing.  The  maximum  current  which  can  be  used  with- 
out giving  loose  or  black  deposits  depends  on  the  speed  at 
which  the  cathode  is  rotating  and  on  its  surface.  The  time 
necessary  for  complete  deposition  is  usually  between  ten 
minutes  and  one  hour,  the  wide  limits  being  due  to  the  many 
variable  factors. 

When  deposition  is  complete,  add  a  few  drops  of  hydrogen 
peroxide  or  about  I  gram  of  urea,  and,  without  stopping  the 
current,  pour  or  siphon  off  the  liquid  as  quickly  as  possible. 
Wash  thoroughly  with  water  and  then  twice  with  alcohol. 
Dry  in  the  steam- oven  for  a  few  minutes,  and  weigh. 

Until  experience  as  to  the  time  necessary  for  complete 
deposition  has  been  gained,  the  solution  must  be  concentrated 
to  about  1 20  c.c.  and  again  electrolysed.  If  nickel  is  absent, 
it  is  more  convenient  to  test  for  traces  of  copper  by  adding 
excess  of  ammonia  to  a  small  portion  of  the  solution.  If 
there  is  still  copper  in  the  solution,  the  test  portion  must  be 
evaporated  nearly  to  dryness  and  added  to  the  main 
portion. 

1  The  small  motors  supplied  by  Fritz  Kohler  (Leipzig)  are  well 
adapted  to  this  class  of  work. 


ELECTROLYTIC  METHODS  153 

Electrolytic  Determination  of  Nickel. 

If  copper  is  present  in  the  original  solution,  it  must  be 
removed  by  depositing,  as  already  described.  If  the  solution 
is  strongly  acid,  no  nickel  is  deposited  with  the  copper.  If 
nitrate  is  present,  it  must  be  removed  by  evaporation  with 
concentrated  sulphuric  acid.  Chloride  and  sulphate  do  not 
interfere  with  the  process. 

Procedure. — The  amount  of  substance  taken  should 
preferably  be  such  as  will  yield  about  0-3  gram  of  nickel. 
Add  5  grams  of  ammonium  sulphate  and  20  c.c.  of  con- 
centrated ammonia,  and  dilute  to  about  150  c.c. 

The  arrangement  of  the  apparatus  should  be  exactly  as 
described  above  for  the  determination  of  copper  with  a 
rotating  cathode,  except  that  the  beaker  containing  the 
solution  must  be  supported  above  a  wire  gauze. 

By  means  of  a  small  flame,  raise  the  temperature  of  the 
solution  to  60° — 80°,  and  keep  it  between  these  limits  during 
the  electrolysis.  The  potential  should  be  from  3  to  4  volts 
(two  accumulator  cells)  and  the  initial  current  about  0-5 
ampere.  Increase  the  current  gradually  to  about  2  amperes. 
When  the  solution  is  colourless,  test  for  nickel  in  a  small 
portion  (5  c.c.)  by  addition  of  hydrogen  sulphide. 

When  all  the  nickel  is  deposited,  siphon  off  the  liquid 
without  stopping  the  current.  Wash  with  dilute  ammonia, 
then  with  water,  and  finally  with  alcohol.  Dry  the  electrode 
in  the  steam-oven,  and  weigh. 

Exercise. — Determine  the  percentages  of  nickel  and  copper 
in  a  German  "  nickel "  coin.  The  coin  should  be  cut  with 
shears  and  about  0-6  gram  used  for  the  analysis. 

Electrolytic  Determination  of  Lead  as  Dioxide. 

The  conditions  of  electrolysis  are  so  adjusted  that  the 
lead  is  deposited  as  lead  dioxide  on  the  anode.  The  only 
metal  which  interferes  with  this  method  is  manganese. 

Procedure. — Clean,  dry,  and  weigh  a  platinum  basin. 
Measure  the  solution  into  the  basin,  add  15  c.c.  of  con- 
centrated nitric  acid,  and  dilute  to  100  c.c.  Connect  with  an 
accumulator,  through  an  adjustable  resistance  and  an  ammeter, 


154  GRAVIMETRIC  ANALYSIS 

making  the  dish  the  anode,  and  pass  a  current  of  about  0-05 
ampere  for  twelve  to  fourteen  hours,  or  overnight.  Then, 
without  interrupting  the  current,  remove  the  acid  liquid  by 
means  of  a  siphon,  and  at  the  same  time  run  distilled  water 
into  the  dish  from  a  tap-funnel,  care  being  taken  that  the 
deposit  of  lead  dioxide  remains  under  the  surface  of  the  liquid 
during  the  operation. 

Continue  washing  until  the  liquid  is  practically  free  from 
acid,  then  stop  the  current,  rinse  the  dish  with  distilled  water, 
and  dry  in  the  air-oven  at  1 80°  to  constant  weight.  The  lead 
dioxide  retains  traces  of  water  even  at  1 80°  and  the  results 
are  somewhat  high.  It  is  advisable,  therefore,  to  heat  the 
dish  gently  in  order  to  convert  the  lead  dioxide  into  lead 
monoxide,  taking  care  to  avoid  contact  with  a  reducing  flame, 
and  then  to  weigh  again. 

The  electrolysis  may  be  completed  in  a  shorter  time  by 
heating  the  solution  to  about  60°,  and  using  a  current  of 
i  to  1-5  ampere. 


PART    IV 

COLORIMETRIC    METHODS 

THE  accurate  determination  of  a  very  small  quantity,  or  of 
the  merest  trace,  of  a  substance  is  frequently  of  the  highest 
importance,  and  it  is  often  found  that  the  most  accurate  and 
by  far  the  easiest  method  is  a  colorimetric  one.  Colorimetric 
methods  are  based  on  a  very  simple  principle  which  the 
following  example  may  serve  to  illustrate. 

When  ammonium  thiocyanate  is  added  to  a  solution  of 
a  ferric  salt,  a  red  coloration  is  produced.  The  colour 
is  perceptible  when  the  solution  contains  even  less  than 
i  part  of  iron  in  10,000,000  (o-i  mgrm.  per  litre),  and  becomes 
more  intense  as  the  quantity  of  iron  is  increased.  The 
amount  of  iron  present  can  be  ascertained  by  finding 
how  much  of  a  very  dilute  standard  solution  of  a  ferric  salt 
must  be  added  to  a  known  quantity  of  water  in  order  to 
produce,  with  ammonium  thiocyanate,  a  coloration  of  equal 
intensity. 

A  colorimetric  determination  occupies  but  a  few  minutes, 
whereas  gravimetric  or  volumetric  methods,  if  applicable 
at  all,  would  prove  extremely  tedious  and  probably  less 
accurate.  Colorimetric  methods  in  general  are  suitable  for 
the  determination  of  small  quantities  only.  Unless  the  total 
amount  of  substance  present  is  known  to  be  small,  the 
solution  containing  it  should  be  diluted  to  a  known  volume 
and  a  suitable  portion  taken  for  the  analysis. 

In  colorimetric  determinations  it  is  essential  (i)  that  the 
solutions  to  be  compared  contain  as  far  as  possible  the  same 
quantities  of  admixed  substances  (e.g.,  acid),  (2)  that  they 
are  at  the  same  temperature,  and  (3)  that  they  are  diluted 
to  the  same  volume  before  adding  the  reagent ;  otherwise, 
serious  errors  may  arise. 

155 


156  COLORIMETRIC  METHODS 

A  number  of  examples  of  the  application  of  colorimetric 
methods  will  be  found  in  Part  VIII.  (Water  Analysis). 

Apparatus. — Special  instruments,  known  as  colorimeters, 
are  sometimes  used  for  measuring  or  comparing  the  intensity 
of  the   colours,   but   for   most   ordinary   purposes   cylinders 
of  colourless    glass    (Nessler    tubes)    are    equally 
satisfactory.      These  tubes  are  graduated  to  con- 
tain   100  c.c.  (or  50  c.c.),  and    must   be  of  equal 
diameter.     The  graduation  mark  must  be  at  the 
same    level    in    all   tubes  of  equal   capacity.      It 
is  worth  while  providing  the  tubes  with  opaque, 
cylindrical  covers,   open   at   the    ends   and  made 
from  stout  brown  paper,  in  order  to  exclude  side 
light.    A   glass   tube,  on    which   a   flattened  bulb 
FIG.  47.       °f  appropriate  size   is  blown  (Fig.  47),  should  be 

provided  for  mixing  the  solutions  in  the  tubes. 
The  general  method  of  procedure  is  fully  described  in  the 
first  example  given  below. 

Iron. 

A  convenient  method  for  the  colorimetric  deter- 
mination of  iron  is  based,  as  already  stated,  on  the  red 
coloration  which  ammonium  thiocyanate  gives  with  a  solution 
of  a  ferric  salt.  The  solution  containing  the  iron  must  be 
acid,  and  a  large  excess  of  the  thiocyanate  must  be  used. 

The  following  solutions  are  required  : — 

(1)  Standard  Iron    Solution. — Dissolve    0-8636   gram    of 
ammonium    ferric  sulphate  in  water,  add    10  c.c.  of  dilute 
hydrochloric  acid,  and  dilute  to  I  litre.    One  c.c.  of  the  solution 
corresponds  to  01  mgrm.  of  iron. 

(2)  Ammonium  Thiocyanate  Solution. — Dissolve  10  grams 
of  the  pure  salt  in  100  c.c.  of  water. 

Procedure. — The  iron  must  be  present  as  a  ferric  salt.  If 
the  solution  contains  more  than  0-3  mgrm.  of  iron,  dilute  it 
to  a  known  volume. 

Measure  into  a  100  c.c.  Nessler  tube  a  portion  of  the 
solution  containing  from  01  to  0-3  mgrm.  of  iron.  (A 
preliminary  trial  may  have  to  be  made  with  a  small  portion.) 
Add  I  c.c.  of  concentrated  hydrochloric  acid  (free  from  iron) 


IRON  157 

and  dilute  to  the  graduation  mark.  Then  add  5  c.c.  of  the 
ammonium  thiocyanate  solution,  and  mix.  If  the  resulting 
coloration  is  very  intense,  use  a  smaller  quantity  of  the  iron 
solution. 

Prepare  four  standard  tints  as  follows  : — Measure  with  a 
burette  0-5,  I,  2,  and  3  c.c.  of  the  standard  iron  solution  into 
four  100  c.c.  tubes,  add  I  c.c.  of  concentrated  hydrochloric 
acid  to  each,  dilute  to  the  mark,  add  5  c.c.  of  the  thiocyanate, 
and  mix. 

Compare  the  intensity  of  the  colour  given  by  the  unknown 
solution  with  the  standard  tints.  The  comparison  is  made 
by  holding  the  tubes  close  together  over  a  white  surface 
(e.g.,  a  sheet  of  opal  glass),  and  looking  down  into  the  tubes. 
It  is  unlikely  that  the  colour  will  match  any  of  the  standards 
exactly,  but  is  now  easy  to  estimate,  and  then  by  actual  trial 
to  ascertain,  what  volume  of  the  standard  iron  solution  must 
be  used  to  give  a  coloration  of  the  same  intensity. 

To  most  observers,  the  solution  in  the  tube  held  in  the 
left  hand  appears  slightly  darker  in  colour  than  that  in  the 
right,  even  when  the  two  tubes  are  filled  with  portions  of  the 
same  solution.  It  is  a  good  plan,  therefore,  in  colorimetric 
comparisons  generally,  to  interchange  the  tubes,  and  if  the 
left-hand  tube  always  appears  the  darker,  it  is  certain  that 
the  intensities  of  the  colorations  are  equal. 

The  following  method  of  matching  may  also  be  used  : — 
Suppose,  for  example,  that  the  unknown  solution  gives 
a  coloration  which  is  a  little  more  intense  than  a  standard 
containing  i  c.c.  of  the  iron  solution.  Transfer  the  unknown 
solution  to  a  measuring  cylinder  (its  volume  is  105  c.c.) ;  then 
pour  it  gradually  back  into  the  Nessler  tube  until  the  colour 
(as  viewed  from  above)  matches  that  of  the  standard.  If  95 
c.c.  is  required,  the  iron  in  the  portion  of  solution  taken  is 

equal  to  that  contained  in  I  x  — -  =  i-i  c.c.  of  the  standard 

iron  solution,  i.e.,  o-i  i  mgrm. 

If  the  quantity  of  iron  taken  requires  more  than  about 
3  c.c.  of  the  standard  solution  to  equal  it,  the  colour  is  too 
deep  for  accurate  comparison. 

The    blue    coloration    which    a    ferric .  salt   gives    with 


153  COLORIMETRIC  METHODS 

potassium  ferrocyanide  may  also  be  used  to  determine  iron 
colorimetrically.  The  colour  varies,  however,  from  blue 
to  pale  green,  according  to  the  nature  and  amount  of  the 
acid  present,  and  the  full  intensity  of  the  colour  is  not  always 
produced  immediately  the  reagent  is  added.  The  method  is 
therefore  less  satisfactory  than  that  described  above. 

Exercise.  —  Determine  the  amount  of  ferric  salt  in  a 
commercial  sample  of  ferrous  sulphate  or  ferrous  ammonium 
sulphate.  Dissolve  a  weighed  quantity  (5  to  I o  grams)  in  water 
to  which  10  c.c.  of  dilute  sulphuric  has  previously  been  added, 
and  dilute  the  solution  to  250  c.c.  Ascertain  by  trial  how 
much  of  the  solution  must  be  diluted  to  100  c.c.  in  order  to 
give,  with  ammonium  thiocyanate,  a  coloration  of  suitable 
intensity  for  comparison. 

Copper. 

When  potassium  ferrocyanide  is  added  to  a  very  dilute 
solution  of  a  copper  salt,  a  purple-brown  coloration  is 
produced.  The  test  is  more  delicate  if  the  solution  contains 
a  large  quantity  of  some  neutral  salt,  such  as  ammonium 
nitrate.  One  part  of  copper  in  about  2,000,000  parts  of 
water  can  be  detected.  The  presence  of  iron  interferes  with 
the  test,  but  lead,  if  not  present  in  large  quantity,  does  not. 
The  following  solutions  are  required  : — 

(1)  Standard  Copper  Solution. — Dissolve  0-3926  gram  of 
pure  copper  sulphate  in  water  and  dilute  the  solution  to  I 
litre.     One  c.c.  of  the  solution  corresponds  too- 1    mgnn.  of 
copper. 

(2)  Potassium  Ferrocyanide  Solution. — Dissolve  I  gram  in 
100  c.c.  of  water. 

(3)  Ammonium  Nitrate  Solution. — Dissolve  10  grams  in 
100  c.c.  of  water. 

Procedure. — The  solution  containing  the  copper  must  be 
neutral.  If  iron  is  present  it  must  be  removed  as  follows  : — 
Add  i  c.c.  of  concentrated  nitric  acid  and  evaporate  to  a 
small  volume.  Precipitate  the  iron  by  adding  a  slight 
excess  of  ammonia,  filter,  and  wash.  Dissolve  the  ferric 
hydroxide  in  dilute  nitric  acid,  reprecipitate  with  ammonia, 


AMMONIA  159 

filter,    and    wash.      Combine   the    two    filtrates,   boil    until 
free  from  ammonia,  cool,  and  dilute  to  a  known  volume. 

Measure  a  suitable  portion  of  the  solution  (corresponding 
to  about  0-5  mgrm.  of  copper)  into  a  100  c.c.  Nessler  tube, 
add  5  c.c.  of  the  ammonium  nitrate  solution,  and  dilute  to 
the  mark.  Then  add  I  c.c.  of  the  potassium  ferrocyanide 
solution,  and  mix.  If  the  coloration  is  not  of  a  suitable 
intensity  for  accurate  comparison,  a  larger  or  a  smaller 
quantity  of  the  copper  solution  should  be  taken.  Then  find 
by  trial,  as  described  in  the  case  of  iron,  how  much  of  the 
standard  copper  solution  is  required  to  give,  under  the  same 
conditions,  a  coloration  of  equal  intensity. 

Copper  may  be  determined  colorimetrically  in  presence 
of  iron  by  means  of  hydrogen  sulphide,  as  described  under 
lead,  provided  that  lead  and  other  metals  forming  sulphides 
insoluble  in  acid  are  absent. 

Ammonia. 

It  is  sometimes  necessary  to  determine  with  accuracy  a 
very  much  smaller  quantity  of  ammonia  than  can  be  dealt 
with  by  the  ordinary  volumetric  method.  In  drinking  water, 
for  example,  I  part  of  ammonia  in  20,000,000  (0-05  mgrm. 
per  litre)  is  of  importance  from  a  hygienic  standpoint.  The 
detection  and  quantitative  determination  of  a  trace  of 
ammonia  so  minute  is  possible  by  means  of  Nessler's 
reagent.  This  reagent  is  a  mixture  of  potassium  mercuric 
iodide  (K2HgI4)  and  sodium  or  potassium  hydroxide,  and 
it  gives  a  brownish-yellow  coloration  with  extremely  dilute 
ammonia  solutions.  On  account  of  the  delicacy  of  the  test, 
and  of  the  small  amounts  of  ammonia  usually  dealt  with,  the 
colorimetric  determination  of  ammonia  must  be  carried  out 
in  a  room  which  contains  no  ammonia  or  ammonium  salts, 
and  not  in  the  general  laboratory. 

The  following  materials  are  required  : — 

(i)  Nessler  Solution. — Dissolve  (a)  35  grams  of  potassium 
iodide  in  150  c.c.  of  water  ;  (b]  17  grams  of  mercuric  chloride 
in  300  c.c.  of  water;  and  (c)  120  grams  of  sodium  hydroxide 
in  300  c.c.  of  water.  Add  (ft)  to  (a)  gradually,  whilst  shaking, 
until  a  slight  red  precipitate  remains  permanent ;  then  add 


160  COLOR1METRIC  METHODS 

(V)  and  dilute  the  mixture  to  I  litre.  Finally,  add  a  little  of 
the  mercuric  chloride  solution  until  a  slight  permanent 
turbidity  again  forms.  Set  the  mixture  aside  until  clear,  and 
then  decant  into  a  bottle  fitted  with  a  rubber  stopper. 

Transfer  a  portion  of  the  solution  to  a  small  bottle  for 
immediate  use  (Fig.  21,  p.  46).  The  pipette  for  measuring 
the  solution  is  graduated  to  deliver  about  2  c.c.  In  order 
that  the  slight  sediment  which  is  sometimes  present  may  not 
be  disturbed,  the  pipette  should  not  reach  to  the  bottom  of 
the  bottle. 

(2)  Standard    Ammonium     Chloride    Solution. — Dissolve 
3-14  grams  of  pure  ammonium  chloride  in  water  and  dilute 
the  solution  to    I    litre.     As  required,  dilute  10  c.c.  of  this 
solution  to  I  litre.     One  c.c.  of  the  dilute  solution  corresponds 
to  ooi  mgrm.  of  ammonia. 

(3)  Ammonia-free     Water.  —  Ordinary    distilled     water 
frequently    contains    a    trace    of    ammonia.      In   order    to 
ascertain  if  this  is  the  case,  mix  50  c.c.  of  the  water  with 
2  c.c.  of  Nessler  solution.     If  no  yellow  coloration  develops 
within  three  minutes,  the  water  is  fit  for  use.     As  a  rule,  it  is 
necessary  to   prepare  ammonia-free  water  in   the  following 
way  : — 

Add  about  I  gram  of  recently  ignited  sodium  carbonate 
crystals  to  about  2  litres  of  distilled  water  contained  in  a 
large  flask  or  copper  boiler.  Distil  the  water  and  test  50  c.c. 
of  the  distillate  from  time  to  time  with  Nessler  solution.  As 
soon  as  it  ceases  to  give  a  coloration,  collect  the  water  in  a 
clean  Winchester.  (Stop  the  distillation  when  the  volume  of 
water  in  the  flask  is  reduced  to  about  250  c.c.)  Keep  the 
Winchester  stoppered  and  away  from  sources  of  ammonia. 

Procedure. — If  the  solution  to  be  examined  contains 
more  than  i  mgrm.  of  ammonia  per  litre,  dilute  it  to  a 
known  volume  with  ammonia-free  water.  If  it  contains  less 
than  02  mgrm.  per  litre,  or  if  any  salts  which  form  insoluble 
hydroxides  are  present,  distil  the  solution  with  a  little  sodium 
carbonate  ;  all  the  ammonia  is  thus  obtained  in  the  first 
portion  of  the  distillate  (cf.  "  Water  Analysis,"  p.  293). 

Measure  into  a  50  c.c.  Nessler  tube  a  portion  of  the 
solution  containing  not  more  than  01  mgrm.  of  ammonia, 


LEAD  161 

and  dilute  it  to  the  mark  with  ammonia-free  water.  Add  2 
c.c.  of  Nessler  solution  and  mix  with  the  stirring-bulb.  (The 
stirring-bulb  must  not  be  laid  on  the  bench  but  should  be 
kept  in  a  beaker  containing  distilled  water.)  Then  ascertain, 
by  trial,  how  much  of  the  standard  ammonium  chloride 
solution  must  be  diluted  to  50  c.c.  (with  ammonia-free  water) 
to  give  a  coloration  of  equal  intensity.  The  full  intensity  of 
the  colour  is  not  obtained  immediately  the  reagent  is  added, 
and  an  interval  of  three  minutes  should  elapse  before  the 
solutions  are  compared. 

Notes. — The  Nessler  solution  must  always  be  added  to 
the  solution  containing  ammonia,  not  vice  versa.  If  the 
solution  contains  more  than  01  mgrm.  of  ammonia  in  50  c.c., 
the  coloration  is  too  intense  for  accurate  comparison.  The 
coloration  corresponding  to  2  or  3  c.c.  of  the  standard 
ammonium  chloride  solution  is  the  most  suitable. 

Lead. 

The  most  satisfactory  method  for  the  determination  of  a 
minute  quantity  of  lead  depends  on  the  coloration  produced 
when  hydrogen  sulphide  is  added  to  the  solution  containing 
it.  The  solution  should  be  slightly  acid — preferably  with 
acetic  acid.  Copper  and  other  metals  which  form  sulphides 
insoluble  in  acid  interfere,  but  dilute  solutions  of  iron  salts 
give  no  coloration  with  hydrogen  sulphide  in  presence  of  acid. 

The  following  solutions  are  required  : — 

(1)  Standard  Lead  Solution. — Dissolve   0-1830  gram  of 
lead  acetate  in  water,  add  acetic  acid  until  a  clear  solution  is 
obtained,   and   dilute   to   I    litre.     One  c.c.  of  the   solution 
corresponds  to  o-i  mgrm.  of  lead. 

(2)  Hydrogen  Sulphide  Solution. — Saturate  some  freshly 
boiled  (and  cooled)  distilled  water  with  the  gas,  transfer  the 
solution  to  a  burette,  and  cover  it  with  about  I  c.c.  of  olive 
oil.      Protected  in  this  way  from   the  air,  the   solution  will 
remain  clear  for  a  long  time,  especially  if  kept  in  the  dark. 

Procedure. — If  the  solution  to  be  tested  is  strongly  acid, 
neutralise  the  excess  of  acid  with  sodium  hydroxide  and 
then  add  a  little  sodium  acetate.  If  it  contains  more  than 
0-5  mgrm.  of  lead,  dilute  it  to  a  known  volume. 

L 


162  C6LORIMETRIC  METHODS 

Measure  into  a  100  c.c.  Nessler  tube  a  portion  of  the 
solution  containing  from  01  to  0-5  mgrm.  of  lead,  add  2  c.c. 
of  acetic  acid,  and  dilute  to  100  c.c.  Then  add  2  c.c.  of 
hydrogen  sulphide  solution  and  mix  gently.  Vigorous 
agitation  may  cause  precipitation  of  the  lead  sulphide.  (In 
order  to  be  sure  of  obtaining  a  clear  brown  coloration  and 
not  a  precipitate,  10  c.c.  of  a  concentrated  solution  of  sugar 
may  be  mixed  with  the  solution  before  adding  the  hydrogen 
sulphide.) 

Then  find,  by  trial,  how  much  of  the  standard  lead 
solution  is  required  to  give,  under  the  same  conditions,  a 
coloration  of  equal  intensity.  The  coloration  gradually  fades 
if  it  is  exposed  to  full  daylight. 

Manganese. 

The  gravimetric  determination  of  a  small  amount  of 
manganese  in  a  complex  substance  is  seldom  accurate, 
largely  owing  to  the  difficulty  of  completely  separating  it 
from  the  other  constituents  of  the  substance.  In  the  majority 
of  cases,  it  is  possible  to  determine  the  manganese  in  a 
separate  portion  of  the  substance  by  a  colorimetric  method, 
and  the  results  are  very  exact. 

The  method  depends  on  the  conversion  of  the  manganese 
into  permanganic  acid  by  means  of  ammonium  persulphate 
in  presence  of  a  small  quantity  of  silver  nitrate.  (If  no 
silver  salt  is  present,  the  manganese  is  precipitated  as  man- 
ganese dioxide.)  The  solution  in  which  the  manganese  is 
to  be  determined  must  contain  nitric  acid  or  sulphuric  acid, 
but  no  chloride.  The  method  is  very  convenient  for  the 
determination  of  manganese  in  iron  or  steel. 

The  following  solutions  are  required  : — 

(1)  Standard  Manganese  Solution. — Dissolve  0-144  grams 
of  pure  potassium  permanganate  in  about  100  c.c.  of  water 
and  pass  a  current  of  sulphur  dioxide  through  the  solution 
until   it   becomes   clear   and   colourless.     Boil   the   solution 
until  free  from  sulphur  dioxide,  cool,  and  dilute  to   i  litre. 
One    c.c.    of   the    solution    corresponds   to   0-05   mgrm.  of 
manganese. 

(2)  Silver  Nitrate   Solution. — Dissolve  i  gram  of  silver 
nitrate  in  500  c.c.  of  water. 


MANGANESE  163 

Procedure. — If  the  solution  contains  more  than  I  mgrm. 
of  manganese,  dilute  it  to  a  known  volume ;  if  it  contains 
less  than  I  mgrm.,  evaporate  the  solution  to  about  50  c.c. 

Transfer  the  solution,  or  a  measured  portion  of  it,  con- 
taining about  i  mgrm.  of  manganese,  to  a  100  c.c.  graduated 
flask,  and  add  10  c.c.  of  the  silver  nitrate  solution.  (If  a 
trace  of  chloride  is  present  and  a  turbidity  appears,  shake 
vigorously  to  coagulate  the  silver  chloride  and  filter  into 
another  flask.)  Then  add  I  gram  of  ammonium  persulphate 
and  warm  the  flask  on  the  steam-bath  until  the  pink  colour 
appears.  After  about  a  minute,  remove  the  flask.  When 
the  colour  has  fully  developed,  cool  the  solution  by  placing 
the  flask  in  cold  water.  Dilute  the  solution  to  the  graduation 
mark. 

Measure  into  another  similar  flask  a  portion  of  the 
standard  manganese  solution  containing  approximately  the 
same  amount  of  manganese  as  in  the  unknown  solution, 
oxidise  with  silver  nitrate  and  ammonium  persulphate  in 
the  same  manner  as  before,  and  dilute  to  the  graduation 
mark. 

Transfer  50  c.c.  of  the  lighter  coloured  solution  to  a 
Nessler  tube,  and  pour  the  darker  solution  into  a  burette. 
Run  the  darker  solution  into  another  Nessler  tube  until  the 
colours  of  the  two  solutions,  as  viewed  from  above,  are  of 
equal  intensity. 

If  40  c.c.  of  the  standard  solution  is  required  to  match 
50  c.c.  of  the  unknown  solution,  the  concentration  of  the 
solution  is  ^%  times  that  of  the  standard.  The  amount  of 
manganese  can  then  be  calculated. 

Exercise. —  Determine  the  percentage  of  manganese  in  a 
commercial  sample  of  lime.  Before  commencing  the  deter- 
mination, make  a  rough  experiment,  using  about  I  gram  of 
the  lime,  in  order  to  ascertain  approximately  how  much 
manganese  is  present.  Dissolve  the  lime  in  nitric  acid. 

Determination  of  Manganese  in  Steel. — Weigh  accur- 
ately 02  gram  of  the  sample  of  steel  (in  the  form  of  clean 
drillings)  and  of  a  standard  steel  in  which  the  percentage  of 
manganese  is  known.  Place  the  weighed  portions  in  100 
c.c.  graduated  flasks.  Add  to  each  flask  10  c.c.  of  a  mixture 


164  COLORIMETRIC  METHODS 

of  concentrated  nitric  acid  and  water  in  equal  volumes,  and 
warm  the  flasks  on  the  steam -bath  until  the  steel  has  dis- 
solved and  all  oxides  of  nitrogen  are  driven  off.  Add  10 
c.c.  of  the  silver  nitrate  solution  and  then  I  gram  of  ammonium 
persulphate,  and  warm  the  flasks  on  the  steam-bath  until 
the  oxidation  commences.  When  the  colour  has  fully 
developed,  cool  the  solutions  and  dilute  to  the  graduation 
mark. 

Measure  10  c.c.  of  the  standard  steel  solution  into  a 
graduated  "  carbon  "  tube,  and  dilute  to  some  convenient 
volume,  e.g.t  15  or  20  c.c.  Into  another  similar  tube  measure 
10  c.c.  of  the  solution  of  the  sample.  (If  the  standard  solu- 
tion is  darker  in  colour  than  that  of  the  sample,  the  standard 
must  be  further  diluted,  or  another  standard  containing  less 
manganese  must  be  prepared.) 

Then  add  water,  little  by  little,  to  the  tube  containing 
the  sample,  mixing  thoroughly  after  each  addition  of  water, 
until  the  colour  matches  that  of  the  standard  solution.  The 
comparison,  in  this  case,  is  made  by  holding  the  tubes  in 
front  of  a  piece  of  thin  paper  (or  a  piece  of  ground  glass) 
which  is  held  towards  the  light.  To  most  observers,  a  good 
match  is  obtained  when,  on  interchanging  the  tubes,  the 
left-hand  tube  always  appears  slightly  darker  than  the  other. 

If  the  steel  contains  more  than  0-75  per  cent,  of  man- 
ganese, use  only  o  i  gram. 

Example. — A  standard  steel  contained  0-3  per  cent,  of 
manganese,  and  it  was  diluted  in  the  graduated  tube  to 
15  c.c. — each  cubic  centimetre  then  corresponded  to  0-02 
per  cent.  The  volume  of  the  solution  of  the  sample  when 
the  colours  matched  was  20  c.c. ;  the  percentage  of  manganese 
in  the  sample  was  therefore  20  x  0-02  =  0-4  per  cent. 


PART   V 
SYSTEMATIC  QUANTITATIVE  ANALYSIS 

ALUMINIUM. 

Neither  a  volumetric  nor  an  electrolytic  method  is  avail- 
able for  the  determination  of  aluminium. 

In  the  analysis  of  a  complex  mixture,  iron  and  aluminium 
are  separated  from  all  other  metals  before  separation  from 
one  another,  and  are  obtained  finally  as  a  mixture  of  ferric 
oxide  and  alumina.  The  iron  in  the  mixed  oxides  may 
be  determined  volumetrically  and  the  aluminium  found  by 
difference. 

Forms  in  which  Aluminium  is  precipitated. 

Aluminium  Hydroxide. — This  is  the  easiest  method  for 
the  determination  of  aluminium,  but  is  limited  in  applicability 
on  account  of  the  general  insolubility  of  metallic  hydroxides. 
For  details  of  the  procedure,  see  p.  130. 

Basic  Aluminium  Acetate. — This  method  serves  to 
separate  aluminium  (andiron)  from  nickel, cobalt,  manganese, 
zinc,  calcium,  and  magnesium. 

Determination  of  Aluminium  (and   Iron)    by    the    Basic 
Acetate  Method. 

OUTLINE  OF  METHOD.— The  solution  is  neutralised  and  largely  diluted. 
Ammonium  acetate  is  added  and  the  solution  boiled,  whereby  basic 
aluminium  and  ferric  acetates  are  precipitated.  After  a  second  pre- 
cipitation by  the  same  method,  the  precipitate  is  ignited  and 
the  mixture  of  Fe2O3  and  A12O3  is  weighed.  The  Fe2O3  in  the  mixed 
oxides  is  then  determined  and  the  A12O3  found  by  difference. 

Basic  Aluminium  Acetate  is  a  bulky,  gelatinous  pre- 
cipitate, insoluble  in  water  or  in  slightly  alkaline  solutions. 
It  probably  varies  considerably  in  composition  according  to 

165 


166          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

the  method  of  preparation.  Unless  precipitated  under  the 
exact  conditions  given  below,  it  forms  a  jelly-like  mass  which 
cannot  be  filtered.  It  is  readily  soluble  in  all  acids.  It 
occludes  to  some  extent  all  soluble  salts  which  may  be 
present  in  the  solution,  and  these  cannot  be  completely 
removed  by  washing.  If  washed  with  water  it  becomes 
more  gelatinous  and  chokes  the  pores  of  the  filter. 

On  ignition  it  is  completely  converted  into  A12O3,  but 
if  the  solution  contained  alkali  salts,  the  precipitated  basic 
acetate  is  contaminated  with  occluded  alkali  salts,  and  the 
oxide  obtained  is  also  impure. 

Basic  Ferric  Acetate  corresponds  closely  in  properties  with 
the  aluminium  salt,  but  is  less  soluble  in  acetic  acid. 

Procedure. — The  basic  acetate  method  is  never  used 
unless  it  is  necessary  to  separate  iron  and  aluminium  from 
other  metals.  In  the  following  description  of  the  method  it 
is  therefore  assumed  that  a  separation  of  iron  and  aluminium 
from  other  metals,  such  as  manganese,  is  desired. 

Introduce  the  solution  into  a  beaker  of  at  least  750  c.c. 
capacity  and,  to  the  cold  solution,  add  ammonia  cautiously, 
with  constant  stirring,  until  a  slight  permanent  precipitate  is 
produced.  Then  add  dilute  hydrochloric  acid,  drop  by  drop, 
stirring  for  about  a  minute  after  each  drop,  until  the  pre- 
cipitate is  just  redissolved.  The  solution  at  this  stage  should 
be  a  clear  brown  colour — if  it  is  yellow,  too  much  acid  has 
been  added.  Add  5  grams  of  solid  ammonium  acetate.  If 
the  conditions  have  been  correctly  observed,  no  precipitate 
will  form  at  this  stage,  but  the  solution  will  become  darker 
and  redder  in  colour.  Dilute  with  hot  water  to  about  400  c.c. 
and  heat  until  boiling.  Boil  for  one  or  two  minutes  only,  and 
filter  the  solution  as  quickly  as  possible  through  an  1 1  cm.  paper , 
using  slight  suction.  (If  the  solution  is  boiled  for  more 
than  two  minutes,  or  is  allowed  to  cool  before  filtration,  the 
precipitate  becomes  so  gelatinous  that  filtration  is  almost 
impossible.) 

The  precipitate  is  mainly  basic  ferric  and  aluminium 
acetates,  but  contains  also  occluded  salts.  Wash  twice  with 
hot  water  and  then  redissolve  the  precipitate  by  pouring  hot 
dilute  nitric  acid  on  it  without  removal  from  the  filter  paper. 
Wash  the  paper  once  with  hot  dilute  nitric  acid  and  then 


AMMONIUM— ANTIMONY  167 

three  times  with  water.  Neutralise  the  cold  solution  and 
reprecipitate  exactly  as  before,  using  the  same  filter  paper 
for  the  filtration.  Wash  the  precipitate  several  times  with  a 
hot  dilute  solution  (about  2  grams  per  litre)  of  ammonium 
acetate.  (Combine  the  two  filtrates  for  the  determination  of 
manganese,  etc.) 

Ignite  the  wet  precipitate  and  filter  paper,  heating  finally 
to  bright  redness,  and  weigh  the  mixture  of  ferric  oxide  and 
alumina.  Ignite  again  over  a  Meker  burner  or  blowpipe 
until  the  weight  is  constant. 

Separation  of  the  Iron  and  Aluminium. — Dissolve  the 
mixed  oxides  by  fusion  with  acid  potassium  sulphate, 
determine  the  iron  volumetrically  (p.  81),  and  calculate  the 
weight  of  Fe2O3  which  is  equivalent  to  it.  The  difference 
between  this  and  the  weight  of  the  mixed  oxides  is  the 
weight  of  A12O3. 

AMMONIUM. 

Ammonia,  whether  as  free  ammonia  or  as  an  ammonium 
salt,  is  determined  volumetrically.  For  details,  see  p.  59. 

A  colorimetric  method  for  the  determination  of  traces  of 
ammonia  is  described  on  page  159. 

ANTIMONY. 

The  gravimetric  determination  of  antimony  is  a  matter 
of  some  difficulty,  as  in  ordinary  practice  the  problem 
involves  the  separation  of  antimony  from  other  elements, 
such  as  arsenic,  with  similar  chemical  properties. 

In  the  analysis  of  a  complex  mixture,  antimony  is  usually 
precipitated  as  sulphide,  together  with  other  metals  of  the 
copper  and  arsenic  group. 

Antimony  Sulphide. — Hydrogen  sulphide  precipitates  from 
a  hot  solution  of  an  antimonious  salt,  an  orange  precipitate 
of  antimonious  sulphide  which  is  often  contaminated  with 
pentasulphide  or  with  free  sulphur. 

Antimonious  sulphide  is  insoluble  in  water.  The  solu- 
bility is  inappreciable  in  hydrochloric  or  sulphuric  acid 
(saturated  with  hydrogen  sulphide)  up  to  about  2  N  solution. 
No  antimony  is  precipitated  by  hydrogen  sulphide  from  a 
5  N  hydrochloric  aqid  solution,  and  a  complete  separation. 


168          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

from  arsenic  may  be  obtained  by  precipitating  the  arsenic 
from  a  strongly  acid  solution,  preferably  after  addition  of 
2  or  3  grams  of  tartaric  acid. 

Antimonious  sulphide  is  readily  soluble  in  alkali 
hydroxides  or  sulphides,  and  in  ammonium  sulphide. 

It  may  be  obtained  as  anhydrous  Sb2S3  by  drying  between 
200°  and  400°  in  an  atmosphere  of  carbon  dioxide. 


ARSENIC. 

Arsenic  in  the  arsenious  condition  or  as  arsenite  may  be 
determined  by  the  volumetric  method  described  on  p.  88. 

Forms  in  -which  Arsenic  is  precipitated. 

Arsenic  Sulphide. — This  method  gives  accurate  results 
if  all  the  arsenic  is  in  the  pentavalent  state.  The  cold 
solution  should  be  diluted  with  twice  its  own  volume  of 
concentrated  hydrochloric  acid  and  saturated  with  hydrogen 
sulphide.  After  filtration  through  a  Gooch  crucible,  the 
precipitate  should  be  washed,  dried  at  105°  to  110°,  and 
weighed  as  As2S5. 

Magnesium  Ammonium  Arsenate. — The  arsenic  must  be 
present  as  arsenate. 

Determination  as  Magnesium  Ammonium  Arsenate. 

OUTLINE  OF  METHOD. — The  arsenic  is  precipitated  by  "Magnesia 
Mixture "  in  presence  of  a  large  excess  of  ammonia,  dried  at  1 10°, 
and  weighed  as  MgNH4AsO4. 

Magnesium  Ammonium  Arsenate  is  a  white  crystalline 
precipitate,  slightly  soluble  in  water  (1-7  grams  per  litre  at 
15°),  but  insoluble  in  ammonia  solution,  even  if  dilute.  The 
precipitated  salt  contains  six  molecules  of  water  of  crystallisa- 
tion, and  cannot  be  completely  dehydrated  at  100°.  At  110°, 
it  quickly  becomes  anhydrous,  and  at  somewhat  higher  tem- 
peratures begins  to  decompose.  If  heated  too  quickly  there 
may  be  loss  in  the  drying  process  through  the  water  vapour 
mechanically  carrying  away  part  of  the  salt. 

The  following  solution  is  required  for  the  precipitation  : — 
Magnesia  Mixture. — Dissolve  6  grams  of  magnesium  chloride 


ARSENIC— BARIUM  169 

and  7  grams  of  ammonium  chloride  in  water,  add   10  c.c.  of 
concentrated  ammonia,  and  dilute  to  100  c.c. 

Procedure. — Evaporate  the  arsenate  solution  to  100  c.c. 
and  add  I  gram  of  ammonium  chloride.  Run  in,  drop  by 
drop,  20  c.c.  of  magnesia  mixture,  stirring  briskly,  but 
without  touching  the  beaker  with  the  stirring-rod.  Then 
add  15  c.c.  of  concentrated  ammonia  and  set  aside  for  twelve 
hours.  Decant  through  a  Gooch  crucible,  and  wash  the 
precipitate  with  a  dilute  solution  of  ammonia  (10  c.c.  of 
concentrated  ammonia  per  litre).  Bring  the  precipitate  into 
the  crucible,  and  continue  to  wash  with  the  dilute  ammonia 
until  the  filtrate  is  free  from  chloride.  Place  the  crucible  in 
the  air-oven  and  raise  the  temperature  slowly,  drying  finally 
at  110°  to  115°  until  of  constant  weight. 

BARIUM. 
Forms  in  which  Barium  is  Precipitated. 

Barium  Sulphate. — This  is  the  usual  method,  and  serves 
for  the  separation  of  barium  from  almost  all  other  metals. 

Barium  Chromate. — This  method  is  also  convenient  and 
accurate.  Barium  may  be  separated  from  strontium  by 
precipitation  as  chromate  from  a  solution  acidified  with  acetic 
acid. 

Determination  of  Barium  as  Sulphate. 

The  solution  is  made  slightly  acid  with  hydrochloric  acid, 
heated  until  boiling,  and  the  barium  precipitated  by  a  hot 
dilute  solution  of  sulphuric  acid.  In  all  other  respects  the 
procedure  is  identical  with  that  adopted  for  the  determination 
of  sulphate  (see  p.  131). 

Determination  of  Barium  as  Chromate. 

OUTLINE  OF  METHOD. — The  barium  is  precipitated  from  neutral  or 
slightly  acid  solution  by  ammonium  chromate.  The  barium  chromate 
is  filtered  through  a  Gooch  crucible,  washed  with  hot  water,  dried, 
and  weighed  as  BaCrO4. 

Barium  Chromate  is  a  yellow,  finely  divided  precipitate 
which  is  very  slightly  soluble  in  water.  At  18°,  I  litre  dis- 
solves 3-8  mgrms,  of  BaCrO4,  and  the  solubility  is  somewhat 


170          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

greater  at  higher  temperatures.  It  is  much  less  soluble  in 
a  dilute  ammonium  chromate  solution.  It  is  appreciably 
soluble  in  dilute  acetic  acid  and  readily  soluble  in  mineral 
acids.  It  can  be  dried  completely  at  100°,  and  may  be  heated 
to  a  red  heat  without  decomposition.  Heated  with  organic 
matter,  partial  reduction  occurs  with  formation  of  green 
chromic  oxide ;  on  further  ignition  with  free  access  of  air  this 
is  again  converted  into  chromate. 

Procedure. — Neutralise  the  solution  with  ammonia  or 
hydrochloric  acid,  and  add  about  I  c.c.  of  dilute  acetic  acid. 
Heat  the  solution  until  boiling,  and  precipitate  with  a  hot 
dilute  solution  of  ammonium  chromate.  (The  chromate  must 
be  free  from  sulphate.  If  it  contains  sulphate,  add  a  few 
drops  of  barium  chloride  solution,  boil,  cool,  and  filter  before 
use.)  Keep  the  solution  for  about  twenty  minutes  before 
filtration,  filter  through  a  Gooch  crucible,  and  wash  about 
eight  times  with  hot  water.1 

Place  the  Gooch  crucible  inside  a  nickel  crucible  and  heat 
with  a  small  flame  for  fifteen  minutes.  Weigh  the  BaCrO4 
obtained. 

BISMUTH. 

In  the  preparation  of  a  solution  for  analysis  it  is  advisable, 
if  possible,  to  use  nitric  acid  in  preference  to  hydrochloric  or 
sulphuric  acid,  as  this  simplifies  the  subsequent  analysis.  In 
the  case  of  a  complex  ore  or  alloy,  it  is  immaterial  which  acid 
is  used,  as  it  is  usually  necessary  to  precipitate  the  copper 
and  arsenic  groups  together  as  sulphides,  and  afterwards  to 
separate  these  by  appropriate  methods. 

Electrolytic  methods  for  the  determination  of  bismuth 
have  been  proposed  but  cannot  be  recommended. 

Forms  in  which  Bismuth  is  precipitated. 

Bismuth  Sulphide. — Bismuth  may  be  precipitated  as 
sulphide  together  with  other  members  of  the  copper  and 

1  Barium  chromate  may  be  safely  washed  with  hot  water  so  long  as 
ammonium  chromate  is  present.  As  soon  as  all  the  ammonium  chromate 
is  removed,  the  pure  wash-water  begins  to  dissolve  appreciable  quantities 
of  barium  chromate.  Do  not  wash,  therefore,  longer  than  is  necessary  to 
remove  the  soluble  salts. 


BISMUTH  171 

arsenic  groups.  If  the  bismuth  sulphide  is  to  be  weighed  as 
such,  it  is  desirable  to  precipitate  in  presence  of  sulphuric 
(not  hydrochloric)  acid. 

Basic  Bismuth  Carbonate. — This  method  is  available  in 
presence  of  sodium,  potassium,  and  ammonium  salts  only. 
The  results  are  inaccurate  if  the  solution  contains  sulphate  or 
chloride. 

Metallic  Bismuth. — This  method  is  available  in  presence 
of  zinc,  aluminium,  sodium,  potassium,  and  ammonium  salts. 

Basic  Bismuth  Nitrate. — This  method  may  be  used  to 
separate  bismuth  from  all  other  metals  except  tin,  antimony, 
and  mercury. 

Determination  of  Bismuth  as  Basic  Carbonate. 

OUTLINE  OF  METHOD. — The  bismuth  is  precipitated  by  ammonium 
carbonate  as  basic  bismuth  carbonate,  which  is  converted  into  oxide 
by  ignition  and  weighed  as  Bi2O3. 

Basic  Bismuth  Carbonate  is  a  granular  white  precipitate 
which  is  readily  soluble  in  acids  and  slightly  soluble  in 
ammonia.  If  precipitated  from  a  solution  containing  chloride 
or  sulphate,  it  is  always  contaminated  with  basic  chloride  or 
sulphate.  As  these  basic  salts  are  not  completely  converted 
into  oxide  by  ignition,  the  method  is  only  applicable  in  absence 
of  all  salts  other  than  nitrate. 

Bismuth  Oxide,  Bi2O3,  is  obtained  when  the  basic  nitrate 
or  carbonate  is  ignited.  Complete  decomposition  occurs  on 
ignition  at  a  low  red  heat.  As  the  molten  oxide  attacks 
porcelain,  it  is  inadvisable  to  heat  above  the  temperature  at 
which  the  oxide  just  melts.  The  oxide  is  readily  reduced  to 
metal  by  carbon,  and  the  filter  paper  should  therefore  be  com- 
pletely incinerated  before  the  precipitate  is  brought  into  the 
crucible. 

Procedure. — Dilute  the  solution  with  water  to  about 
50  c.c.  (a  slight  turbidity  may  be  produced  on  dilution). 
Add  ammonium  carbonate  in  slight  excess,  and  boil  until 
most  of  the  ammonia  is  expelled.  Filter,  wash  the  pre- 
cipitate with  hot  water,  and  dry.  Remove  the  precipitate 
from  the  filter  paper,  and  incinerate  the  paper  in  a  porcelain 
crucible  before  addition  of  the  precipitate.  Heat  with  a 


172  SYSTEMATIC  QUANTITATIVE  ANALYSIS 

small  flame,  the  heat  being  so  regulated  that  the  bismuth 
oxide  is  just  fused,  and  weigh  as  Bi2O3. 

Determination  of  Bismuth  as  Oxide  after  Precipitation 

as  Metal. 

OUTLINE  OF  METHOD. — The  bismuth  is  precipitated  as  metal  by 
formaldehyde  and  alkali.  The  precipitated  metal,  which  is  usually 
contaminated  with  alkali,  is  dissolved  in  dilute  nitric  acid  and  re- 
precipitated  as  carbonate.  By  ignition,  the  carbonate  is  converted 
into  the  oxide,  Bi2O3. 

Metallic  Bismuth,  as  obtained  by  precipitation,  is  a  black 
spongy  powder  which  is  insoluble  in  water  or  in  alkaline 
solutions. 

Procedure. — To  the  bismuth  solution,  add  about  10  c.c. 
of  40  per  cent,  formaldehyde  solution  and  excess  of  pure 
10  percent,  sodium  hydroxide  solution.  Warm  on  the  steam- 
bath.  When  the  precipitate  has  settled  and  the  supernatant 
liquid  has  become  clear,  add  a  further  5  c.c.  of  formaldehyde 
and  a  few  cubic  centimetres  of  sodium  hydroxide.  Heat 
the  solution  until  boiling,  and  press  together  with  a  glass 
rod  the  spongy  precipitate  of  metal,  in  order  to  facilitate 
filtration.  Filter,  wash  thoroughly  with  hot  water,  and  then 
dissolve  the  precipitate  with  a  little  hot  dilute  nitric  acid. 
Wash  the  filter  paper  several  times  with  hot  dilute  nitric 
acid.  Precipitate  the  bismuth  as  basic  carbonate  and 
convert  to  oxide,  as  described  above. 

Determination  of  Bismuth  as  Basic  Nitrate. 

OUTLINE  OF  METHOD. — The  bismuth  is  obtained  in  solution  as  nitrate, 
and  precipitated  as  the  basic  nitrate  by  large  dilution  and 
neutralisation.  The  basic  nitrate  is  converted  into  oxide  by  ignition, 
and  weighed  as  Bi2O3. 

Basic  Bismuth  Nitrate  is  usually  a  mixture  of — 
Bi(OH)(N03)2  and  Bi(OH)2(NO3). 

It  is  practically  insoluble  in  cold  water,  and  still  less  soluble 
in  a  very  dilute  solution  of  ammonium  nitrate.  It  is  readily 
soluble  in  acids  unless  extremely  dilute ;  it  is  necessary, 
however,  that  in  the  precipitation  the  solution  should 


BISMUTH— BROMIDE  173 

still  contain  a  trace  of  acid,  otherwise  other  metals  are 
precipitated  with  the  bismuth.  If  precipitated  from  a 
solution  containing  more  than  a  small  amount  of  chloride, 
the  precipitate  is  contaminated  with  bismuth  oxychloride 
which  is  not  completely  converted  into  oxide  by  ignition. 

Bismuth  Oxide. — The  properties  of  bismuth  oxide  are 
described  on  p.  171. 

Procedure.  —  The  bismuth  solution  must  not  contain 
more  than  a  trace  of  chloride ;  if  it  contains  chloride  in 
quantity,  add  5  c.c.  of  concentrated  nitric  acid  and  evaporate 
until  of  syrupy  consistency ;  then  add  a  further  5  c.c.  of  nitric 
acid,  and  again  concentrate  the  solution.  After  this  treat- 
ment, the  solution  will  be  practically  free  from  chloride. 

To  the  bismuth  nitrate  solution,  contained  in  a  700-1000 
c.c.  beaker,  add  500  c.c.  of  water  and  5  c.c.  of  methyl 
orange  solution.  (Partial  precipitation  of  the  basic  nitrate 
usually  occurs  on  dilution.)  Add  ammonia  drop  by  drop, 
with  constant  stirring,  until  the  pink  colour  is  almost,  but 
not  quite,  discharged.  Set  the  solution  aside  for  one  hour 
and  then  filter.  Wash  with  a  dilute  solution  (2  grams  per 
litre)  of  ammonium  nitrate,  dry  at  100°,  remove  the 
precipitate  as  completely  as  possible  from  the  paper,  and 
incinerate  the  paper  before  addition  of  the  precipitate.  The 
ignition  of  the  precipitate  should  be  carried  out  with  the 
crucible  covered,  and  with  a  very  small  flame  at  first,  the 
temperature  being  raised  slowly.  If  the  heating  is  rapid,  the 
gases  given  off  during  the  decomposition  of  the  nitrate 
carry  away  mechanically  some  of  the  solid.  The  final  tem- 
perature should  be  just  sufficient  to  fuse  the  bismuth  oxide. 

BROMIDE. 

Bromide  is  most  readily  determined  volumetrically  (see 
pp.  99  and  103). 

Bromide  may  also  be  determined  gravimetrically  by 
precipitation  as  silver  bromide.  The  procedure  is  identical 
with  that  adopted  for  the  determination  of  chloride  (see  p.  133). 

Silver  Bromide  is  less  soluble  than  the  chloride.  One 
litre  of  water  dissolves  o-i  mgrm.  at  20°  and  7-3  mgrms.  at 
100°.  It  is  insoluble  in  nitric  acid,  sparingly  soluble  in 


174          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

ammonia,  and  appreciably  soluble  in  concentrated  solutions 
of  most  salts.  It  is  darkened,  with  partial  decomposition, 
by  light,  though  to  a  less  extent  than  the  chloride.  It  may 
be  fused  without  decomposition  but  acquires  a  darker  colour. 
It  can  be  dried  completely  at  100°. 

CADMIUM. 

The  separation  of  cadmium  from  certain  metals,  such  as 
mercury,  is  a  matter  of  some  difficulty,  and  for  methods  of 
analysis  applicable  to  complex  ores  or  alloys,  reference  should 
be  made  to  larger  text-books.  The  analysis  of  Wood's  alloy 
(tin,  lead,  cadmium,  and  bismuth)  is  described  on  p.  226. 

Forms  in  which  Cadmium  is  precipitated. 

Metallic  Cadmium  (Electrolytic). — This  is  a  convenient 
and  accurate  method.  For  details,  see  p.  149. 

Cadmium  Sulphide. — This  method  serves  to  separate 
cadmium  from  metals  of  the  iron,  zinc,  and  calcium  groups. 
If  other  members  of  the  copper  group  are  present,  further 
treatment  is  necessary. 

Determination  of  Cadmium  as  Sulphate. 

(After  precipitation  as  Sulphide?) 

OUTLINE  OF  METHOD. — The  solution  is  acidified  with  sulphuric  acid, 
and  the  cadmium  is  precipitated  with  hydrogen  sulphide.  The 
cadmium  sulphide  is  dissolved  in  hydrochloric  acid,  the  solution 
evaporated  to  dryness  with  sulphuric  acid,  and  the  residue,  CdSO4, 
is  weighed. 

Cadmium  Sulphide  is  a  yellow  precipitate,  insoluble  in 
dilute  acids  or  alkalis.  It  is  readily  soluble  in  concentrated 
hydrochloric  acid.  It  may  be  completely  precipitated  by 
hydrogen  sulphide  if  100  c.c.  of  the  solution  contains  10  to 
12  c.c.  of  concentrated  hydrochloric  acid,  or  5  to  7  c.c.  of 
concentrated  sulphuric  acid ;  under  these  conditions  no  zinc 
will  be  precipitated  with  the  cadmium.  The  precipitate  is 
always  contaminated  with  compounds  such  as  Cd2Cl2S  or 
Cd2SO4S,  and  must  therefore  be  converted  into  cadmium 
sulphate  before  weighing. 


CALCIUM— CARBONATE  175 

Procedure. — To  the  cadmium  solution  add  5  c.c.  of 
concentrated  sulphuric  acid,  dilute  to  100  c.c.,  and  saturate 
with  hydrogen  sulphide.  Filter,  and  wash  with  hot  water. 
Dissolve  the  precipitate  in  the  minimum  amount  of  concen- 
trated hydrochloric  acid,  wash  the  filter  with  hot  dilute  acid? 
and  transfer  the  filtrate  and  washings  to  a  large  crucible. 
Add  0-5  c.c.  of  concentrated  sulphuric  acid,  and  evaporate  as 
far  as  possible  on  the  steam-bath.  Place  the  crucible  inside 
a  larger  nickel  crucible,  and  heat  gently  until  no  fumes  of 
sulphuric  acid  are  given  off.  Weigh  the  CdSO4. 

CALCIUM. 

Calcium  may  be  determined  volumetrically  by  the  method 
given  on  p.  69. 

In  gravimetric  analysis,  calcium  is  always  determined, 
after  removal  of  the  copper,  iron,  and  zinc  groups,  by  pre- 
cipitation as  oxalate.  (For  details,  see  p.  143.) 

CARBONATE. 

Carbonate  may  be  determined  either  gravimetrically  or 
volumetrically  in  various  ways.  Two  gravimetric  methods 
are  in  common  use,  viz. : — 

1.  A  direct  method,  in  which  the  carbon  dioxide,  expelled 
from  the  carbonate  by  the  action  of  acid,  is  absorbed  by 
soda-lime  and  weighed. 

2.  An  indirect  method,  in  which  the  loss  of  weight  due 
to  the  escape  of  the  carbon  dioxide  from  an  apparatus  is 
ascertained. 

Direct  Method. 

OUTLINE  OF  METHOD. — A  weighed  quantity  of  the  substance  is  mixed 
with  dilute  acid  in  a  small  flask  connected  with  a  series  of  drying 
tubes,  and  with  two  absorption  tubes  containing  soda-lime.  The 
soda-lime  tubes  are  weighed  before  and  after  the  experiment. 

The  Apparatus  (Fig.  48)  consists  of  the  following  : — 
A.  A  distilling  flask,  of  about  125  c.c.  capacity,  provided 
with  a  rubber  stopper  and  dropping  funnel.     The  stem  of 
the  latter  should  reach  almost  to  the  bottom  of  the  flask,  and 
the  end  should  be  drawn  out  to  a  point  and  up-turned. 


176          SYSTEMATIC  QUANTITATIVE   ANALYSIS 

B.  A    U-tube,   the   open   ends   of  which    are    sealed    in 
the  blowpipe  flame,  containing  just   sufficient   concentrated 
sulphuric  acid  to  close  the  bend. 

C.  A    U-tube   containing   granulated    pumice   which   has 
been  soaked   in  concentrated  copper  sulphate  solution,  and 
afterwards  heated  for  several  hours  in  an  air-oven  at  1 60°  in 
order  to  partially  dehydrate  the  copper  sulphate.     The  object 
of  this  tube   is   to  retain   hydrogen   sulphide   arising   from 
decomposable  sulphides  present  in  the  substance,  and  any 
hydrochloric  acid  that  may  be  carried  over  with  the  carbon 
dioxide. 


FIG.  48. 

D.  A  U-tube  containing  moderately  fine  granular  calcium 
chloride,  free  from  powder.  The  calcium  chloride  is  intro- 
duced through  a  cylinder  of  glazed  paper,  and  the  tube  is 
filled  to  within  2  cm.  of  the  side-tubes.  Loose  wads  of  glass 
wool  are  then  placed  in  each  limb,  any  calcium  chloride 
adhering  to  the  upper  part  of  the  tube  is  removed,  and  the 
taps  are  made  gas-tight  with  the  minimum  quantity  of 
grease  (Fig.  49). 

In  order  to  remove  from  the  calcium  chloride  any  free 
lime  or  basic  chloride,  which  absorb  carbon  dioxide,  a  slow 
current  of  carbon  dioxide  is  passed  through  the  tube  for  five 
minutes  in  order  to  displace  the  air ;  the  outlet  tap  of  the 
U-tube  is  then  closed,  and  the  tube  is  left  attached  to  the 
Kipp  generator  for  several  hours  or  overnight.  The  carbon 


CARBONATE 


177 


dioxide  in  the  tube  is  then   displaced  by  passing  dry  air 
through  it  for  about  fifteen  minutes. 

E.  and  F.  Two  U-tubes  containing  soda-lime1  and 
calcium  chloride.  A  small  wad  of  cotton  wool  is  placed 
near  the  middle  of  one  limb,  and  fine  granular  soda-lime  is 
introduced  through  a  paper  cylinder  so  as  to  fill  about  three- 
fourths  of  the  tube.  The  remaining  fourth  is  filled  with 
granular  calcium  chloride,  and  small  wads  of  glass  wool  are 
placed  in  each  limb  (Fig.  50).  The  absorption  of  carbon 
dioxide  by  soda-lime  takes  place  with  evolution  of  heat,  and 


Calcium  r»* 

Chloride 


Glass  Wool 
Calcium   Chloride 
Cotton  Wool 

Soda   Lime 


FIG.  49. 


FIG.  50. 


loss  of  the  water  which  is  formed  at  the  same  time  is  pre- 
vented by  the  calcium  chloride. 

G.  A  pulsimeter  and  guard-tube,  containing  a  few  drops 
of  concentrated  sulphuric  acid.  The  latter  protects  the 
calcium  chloride  in  the  last  U-tube  from  atmospheric 
moisture,  and  also  shows  the  rate  at  which  air  leaves  the 
apparatus. 

H.  A  tube  containing  soda-lime  which  removes  carbon 
dioxide  from  the  air  that  is  finally  drawn  through  the 
apparatus. 

K.  An  aspirator ;  an  inverted  wash-bottle  with  the  jet 
removed  and  supported  on  a  retort-stand  ring  may  be  used. 

1  Soda-lime  quickly  deteriorates  if  exposed  to  the  air,  and  soon 
becomes  useless  for  the  absorption  of  carbon  dioxide.  For  this  reason 
it  is  best  to  obtain  it  in  small^  well-corked  bottles. 

M 


178          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

In  either  case,  the  flow  of  water  is  regulated  by  means  of 
a  screw-clip. 

The  best  form  of  U-tube  is  provided  with  hollow,  ground- 
in  glass  taps.  If  plain  U-tubes  are  used,  they  may  be  closed 
with  tightly  fitting  rubber  stoppers,  or  with  ordinary  well- 
softened  corks  which  are  cut  off  flush  with  the  top  of  the 
tube,  and  made  gas-tight  by  brushing  over  with  melted 
paraffin  wax. 

The  contents  of  the  U-tubes  must  be  protected  from 
atmospheric  moisture  and  carbon  dioxide.  This  is  accom- 
plished with  the  first  form  of  U-tube,  by  simply  turning  the 
taps ;  plain  U-tubes  must  be  provided  with  caps,  fitted 
over  the  side-tubes,  and  made  from  short  pieces  of  rubber 
tubing  closed  with  plugs  of  glass  rod.  The  U-tubes  are 
supported  by  wire  hooks  attached  to  a  glass  rod  held  in  a 
clamp.  They  are  connected  with  each  other  by  means  of 
short  pieces  of  thick-walled  rubber  tubing  (pressure  tubing), 
which  are  lubricated  by  rubbing  the  inner  surface  with  a 
little  graphite,  any  excess  of  which  is  carefully  removed. 

Procedure. — Carefully  wipe  the  two  soda-lime  absorption 
tubes  and  leave  them  in  the  balance-room  for  fifteen  minutes 
before  weighing.  (Remove  the  rubber  caps  before  weighing.) 
Weigh  the  carbonate  (e.g.,  about  i  gram  of  calcspar)  in  a 
small  tube,  about  I  in.  long  and  J  in.  wide.  Place  the  tube 
and  contents  in  the  distilling  flask,  and  moisten  with  a  few 
drops  of  water. 

Set  up  the  apparatus,  as  shown  in  Fig.  48.  The  U-tubes 
are  attached  one  after  the  other  beginning  with  B,  and  the 
ends  of  the  glass  tubes  should  be  brought  close  together 
inside  each  rubber  junction.  The  absorption  tubes  E  and  F 
must  be  so  placed  that  the  limbs  containing  calcium  chloride 
are  turned  towards  the  aspirator.  The  aspirator  is  not  con- 
nected at  this  stage. 

Test  the  apparatus,  in  order  to  find  if  it  is  gas-tight,  as 
follows  : — Attach  a  piece  of  glass  tubing  to  the  guard-tube  G, 
and  dip  the  tube  into  a  beaker  of  water.  Apply  gentle 
suction  at  H  in  order  to  lift  a  column  of  water  in  the  tube 
attached  to  G,  and  then  close  the  tap  of  the  dropping  funnel. 
The  apparatus  may  be  considered  gas-tight  if  the  level  of  the 
water  in  the  tube  remains  constant  for  several  minutes. 


CARBONATE  179 

Now  place  about  20  c.c.  of  dilute  hydrochloric  acid  in 
the  dropping  funnel,  open  the  tap  carefully,  and  regulate  the 
flow  of  acid  and  the  evolution  of  gas  so  that  about  two 
bubbles  per  second  pass  through  the  acid  in  B. 

After  a  slight  excess  of  acid  has  been  added,  and  when 
the  evolution  of  gas  has  become  slow,  close  the  tap  of  the 
dropping  funnel  and  (by  means  of  a  pipette)  remove  any 
acid  remaining  in  the  latter.  Warm  the  contents  of  the 
flask  very  gradually  with  a  small  flame  until  the  liquid  just 
boils.  Boil  gently  for  about  one  minute,  then  lower  the 
flame  until  boiling  just  ceases.  Attach  the  aspirator  and 
the  soda-lime  tube  H,  and  draw  a  slow  current  of  air  through 
the  apparatus.  As  soon  as  the  first  soda-lime  tube  becomes 
cold,  extinguish  the  flame,  and  continue  the  current  of  air 
for  fifteen  minutes  more. 

Detach  the  soda-lime  tubes  and,  after  wiping  them,  leave 
them  in  the  balance-room  for  about  half  an  hour  before 
weighing.  The  weight  of  the  tube  F  should  remain  practi- 
cally constant,  any  increase  amounting  to  not  more  than 
about  i  mgrm.  A  decided  increase  in  weight  shows  either 
that  the  experiment  was  conducted  too  hurriedly,  or  that 
the  soda-lime  is  unsatisfactory.  In  general,  the  amount 
of  carbonate  taken  for  analysis  should  be  so  chosen 
that  the  increase  in  weight  of  the  soda-lime  tube  is  about 
0-3  gram. 

It  is  usual  to  calculate  the  percentage  of  carbonate  in  the 
substance  as  CO2. 

Indirect  Method. 

OUTLINE  OF  METHOD.— A  weighed  quantity  of  the  substance  is  de- 
composed by  dilute  acid  in  an  apparatus  of  special  design,  and  the 
loss  in  weight  of  the  apparatus,  due  to  the  escape  of  carbon  dioxide, 
is  ascertained. 

This  method  is  not  so  accurate  as  the  direct  method 
(absorption  by  soda-lime),  and  its  use  is  preferably  restricted 
to  the  analysis  of  carbonates  which  can  be  decomposed 
by  dilute  sulphuric  acid.  If  hydrochloric  acid  is  used,  its 
volatility  makes  it  somewhat  difficult  to  prevent  loss  of 
traces  of  that  acid,  and  the  result,  on  this  account,  may  be 
slightly  high. 


i80          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

The  Apparatus  (Fig.  51)  consists  of  a  small,  wide-mouthed 
flask,  of  thin  glass  for  the  sake  of  lightness,  and  of  100  to 
1 20  c.c.  capacity.  The  flask  is  fitted  with  a  rubber  stopper, 
through  which  pass  a  calcium  chloride  drying  tube  and  a  tube 
that  reaches  nearly  to  the  bottom  of  the  flask  and  is  drawn 
to  a  point  at  its  lower  end.  The  drying  tube  is  filled  with 
granular  calcium  chloride  with  glass-wool  plugs  at  either  end, 
and  the  calcium  chloride  must  be  saturated  with  carbon 
dioxide,  as  previously  described  (see  the  direct  method).  If 
the  carbonate  is  to  be  decomposed  by  means  of  hydrochloric 


FIG.  51. 

acid,  one-half  of  the  drying  tube  B  is  filled  with  granular 
pumice  which  had  been  soaked  in  concentrated  copper 
sulphate  solution  and  dried  at  160°,  and  the  other  half  with 
calcium  chloride.  The  acid  required  for  the  decomposition  of 
the  carbonate  is  contained  in  a  small  test-tube,  the  length  of 
which  is  so  adjusted  that  the  tube  will  stand  obliquely  in  the 
flask,  but  cannot  fall  into  a  horizontal  position.  The  tubes 
A  and  C  are  provided  with  rubber  caps  closed  with  short 
pieces  of  glass  rod.  Two  additional  straight  calcium  chloride 
tubes  are  also  required. 

Procedure. — Weigh  the  substance  in  the  dry  flask  (e.g,, 
about  2  grams  of  sodium  carbonate  crystals).  Measure  a 
slight  excess  of  dilute  sulphuric  acid  (5  to  10  c.c.)  into  the 
small  test-tube,  and  fit  the  apparatus  together.  Allow  it  to 


CHLORATE— CHLORIDE  181 

remain  in  the  balance-room  for  fifteen  minutes,  remove  the 
rubber  caps,  and  weigh. 

Replace  the  cap  on  tube  A,  attach  one  of  the  supple- 
mentary calcium  chloride  tubes  to  C  (in  order  to  protect  the 
contents  of  B  from  atmospheric  moisture)  and,  by  carefully 
tilting  the  flask,  allow  the  acid,  a  few  drops  at  a  time,  to  come 
into  contact  with  the  carbonate.  The  evolution  of  carbon 
dioxide  must  not  be  rapid,  otherwise  moisture  will  be  carried 
away  with  the  gas.  When  the  whole  of  the  acid  has  been 
mixed  with  the  carbonate  and  effervescence  has  ceased, 
warm  the  flask  cautiously  with  a  very  small  flame  until  the 
liquid  is  heated  almost  to  the  boiling  point.  Then  attach  an 
aspirator  to  C,  and,  the  aspiration  having  been  started, 
remove  the  cap  on  A,  attach  a  calcium  chloride  tube  to  A, 
and  draw  a  slow  current  of  dry  air  through  the  apparatus  for 
ten  minutes.  Remove  the  flame,  continue  the  air  current 
for  ten  minutes  more,  replace  the  caps  on  A  and  C,  and,  after 
an  interval  of  about  thirty  minutes,  weigh  the  apparatus 
(without  the  caps). 

The  loss  in  weight  represents  the  carbon  dioxide  expelled 
from  the  carbonate  used. 

Aspirator. — An  evacuated  Winchester  quart  bottle,  closed 
with  a  rubber  stopper  through  which  passes  a  tube  provided 
with  a  tap,  makes  a  very  convenient  aspirator. 

CHLORATE. 

Chlorate  is  usually  determined  by  reduction  to  chloride 
and  determination  of  the  chloride  by  one  or  other  of  the 
methods  mentioned  below.  A  convenient  method  of  re- 
duction is  described  on  p.  104.  Another  method  is  to  add 
excess  of  ferrous  sulphate  and  keep  the  solution  near  the 
boiling  point  for  about  twenty  minutes.  The  basic  ferric 
salt  which  separates  is  dissolved  in  nitric  acid  and  the 
chloride  determined. 

CHLORIDE. 

Chloride  may  be  determined  volumetrically  by  the 
methods  given  on  pp.  99  and  103. 

The  gravimetric  determination  of  chloride  is  described 
on  p.  133. 


182          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

CHROMIUM. 

The  volumetric  method  described  on  p.  83  is  convenient 
and  accurate. 

In  order  to  separate  chromium  from  other  metals,  it  is 
first  oxidised  to  chromate  by  fusion  with  sodium  peroxide, 
or  with  a  mixture  of  sodium  carbonate  and  potassium  nitrate  ; 
the  chromate  is  determined  volumetrically  by  the  method 
already  described,  or  gravimetrically,  as  described  on  p.  183. 

In  the  absence  of  all  metals  other  than  the  alkalis, 
chromium  may  be  determined  by  precipitation  as  hydroxide 
and  conversion  into  oxide.  The  procedure  is  identical  with 
that  described  under  Aluminium  on  p.  130. 

Chromic  Hydroxide,  when  freshly  precipitated,  is  a  grey- 
green,  flocculent  substance,  insoluble  in  water.  It  is  readily 
soluble  in  acids  and  in  sodium  hydroxide.  It  is  sparingly 
soluble  in  ammonia,  yielding  a  violet-red  solution  ;  on  boiling 
this  solution,  the  ammonia  is  expelled  and  the  chromic 
hydroxide  is  precipitated.  When  dried  at  100°,  it  loses 
water  of  hydration  and  becomes  bluish-green.  On  gentle 
ignition  it  is  converted  into  chromic  oxide,  Cr2O3. 

Chromic  Oxide  is  a  dark  green  powder  which  may  be 
ignited  strongly  without  loss  of  weight.  The  oxide,  after 
strong  ignition,  is  insoluble  in  hydrochloric  acid. 

Further  information  in  connection  with  the  determination 
of  chromium  will  be  found  below  and  on  p.  83. 

CHROMATE  AND  BICHROMATE. 

Chromate  and  dichromate  are  usually  determined  volu- 
metrically (p.  83).  If  the  method  is  practicable,  the  gravi- 
metric determination  as  mercurous  chromate  is  easy  and 
accurate.  Chromate  (or  dichromate)  may  also  be  determined 
gravimetrically  by  reduction  to  a  chromic  salt,  followed  by 
precipitation  as  chromic  hydroxide  (see  under  Chromium). 

Forms  in  -which  Chromate  is  precipitated. 

Mercurous  Chromate. — This  method  is  applicable  if 
chloride  is  present  only  in  small  amount.  A  large  quantity 
of  sulphate  also  renders  the  method  inaccurate. 


COPPER  183 

Barium  Chromate. — Chloride  does  not  interfere  with 
the  use  of  this  method,  but  sulphate  must,  of  course,  be 
absent.  The  properties  of  barium  chromate  are  described 
on  p.  169. 

Determination  of  Chromate  as  Mercurous  Chromate. 

OUTLINE  OF  METHOD. — The  chromate  is  precipitated  as  mercurous 
chromate,  which  on  ignition  is  decomposed,  leaving  chromic  oxide, 
Cr2O3. 

Mercurous  Chromate. — On  addition  of  mercurous  nitrate 
to  a  chromate,  a  brown  precipitate  of  a  basic  salt  separates. 
This  quickly  changes  to  the  bright  red  normal  salt,  Hg2CrO4. 
Mercurous  chromate  is  insoluble  in  water  and  in  very  dilute 
nitric  acid.  On  ignition,  it  is  converted  into  chromic  oxide 
(for  the  properties  of  chromic  oxide,  see  p.  182).  If  the 
mercurous  chromate  is  contaminated  with  much  mercurous 
chloride,  the  precipitate  is  bulky  and  inconvenient,  and 
chromic  oxide  is  lost  during  the  ignition. 

Procedure. — Add  nitric  acid  until  the  solution  is  neutral 
or  very  slightly  acid.  Heat  the  solution  and  add  mercurous 
nitrate  until  precipitation  is  complete.  Keep  the  solution 
hot  until  the  precipitate  becomes  bright  red.  Then  add 
ammonia  cautiously  until  a  small  quantity  of  a  dark  grey 
precipitate  permanently  forms.  Heat  until  boiling,  then 
cool,  filter,  and  wash  with  a  dilute  mercurous  nitrate  solution. 
Dry  the  precipitate  and  the  filter.  Burn  the  filter  before  the 
addition  of  the  precipitate.  Ignite  very  gently  at  first  and 
in  a  good  draught,  and  finally  ignite  with  the  blowpipe. 
(Caution.  The  vapour  given  ofT  during  the  ignition  of  the 
mercurous  chromate  is  poisonous.)  Cool,  and  weigh  the 
Cr203. 

COPPER. 

Copper  may  be  determined  volumetrically  (see  p.  89). 
In  gravimetric  analysis,  copper  is  precipitated  in  acid 
solution,  together  with  many  other  metals,  by  hydrogen 
sulphide. 

A  colorimetric  method  for  the  determination  of  traces  of 
copper  is  described  on  p.  158. 


184          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Forms  in  which  Copper  is  precipitated. 

Cupric  Sulphide. — Precipitation  is  complete  even  in 
strongly  acid  solution,  and  this  method  is  therefore  used  to 
separate  copper  from  iron,  zinc,  etc.  It  does  not  separate 
copper  from  bismuth,  mercury,  etc.  For  details,  see  p.  141. 

Cupric  Oxide.  —  This  is  a  convenient  and  accurate 
method  but  is  seldom  applicable,  as  no  other  metals  which 
give  insoluble  oxides  may  be  present.  For  details,  see  p.  139. 

Cuprous  Thiocyanate.  —  Since  most  thiocyanates  are 
soluble,  this  method  is  sometimes  useful  for  the  separation 
of  copper  from  other  metals. 

Metallic  Copper  (Electrolytic  method). — This  is  prob- 
ably the  best  method  for  separating  copper  from  the  other 
metals  precipitated  with  it  by  hydrogen  sulphide  in  acid 
solution.  For  details,  see  p.  150. 

Determination  of  Copper  as  Cuprous  Thiocyanate. 

OUTLINE  OF  METHOD. — The  copper  is  precipitated  with  ammonium 
thiocyanate  in  presence  of  sulphurous  acid,  and  is  weighed  as 
cuprous  thiocyanate  after  drying  at  140°  to  150.° 

Cuprous  Thiocyanate^  CuCNS,  is  a  pure  white,  crystalline 
precipitate,  almost  insoluble  in  dilute  hydrochloric,  sulphuric, 
and  sulphurous  acid.  One  litre  of  water  at  1 8°  dissolves  0-5 
mgrm.  of  cuprous  thiocyanate,  but  the  solubility  is  greater 
in  all  acid  and  salt  solutions,  more  particularly  at  higher 
temperatures.  It  may  be  dried  without  decomposition  at 
temperatures  not  exceeding  150°. 

Procedure. — The  solution  should  be  made  slightly  acid 
with  sulphuric  acid.  (Nitrates  or  other  oxidising  agents, 
if  present,  must  be  removed  by  evaporation  with  sulphuric 
acid.)  Add  excess  of  sulphurous  acid,  warm,  and  to  the 
warm  solution  add  ammonium  thiocyanate,  drop  by  drop, 
with  constant  stirring.  The  greenish  precipitate  becomes 
pure  white  when  stirred  for  some  time.  When  the 
precipitate  has  settled  (which  may  be  after  some  hours), 
filter  through  a  Gooch  crucible,  and  wash  with  cold  water 
until  ferric  chloride  gives  no  coloration  with  the  washings. 
Wash  finally  several  times  with  20  per  cent,  alcohol,  dry  at 
a  temperature  not  exceeding  150°,  and  weigh  as  CuCNS, 


IRON  185 

IRON. 

On  account  of  its  rapidity  and  accuracy,  the  volumetric 
determination  of  iron  (p.  76)  is  preferable  to  any  gravimetric 
method. 

The  volumetric  method  may  often  be  applied  after  the 
iron  has  been  separated  gravimetrically  from  most  other 
metals,  and  it  is  one  of  the  best  methods  for  the  deter- 
mination of  the  iron  in  a  mixed  precipitate  of  ferric  and 
aluminium  oxides. 

In  gravimetric  analysis,  iron  is  always  determined  after 
removal  of  the  metals  precipitated  by  hydrogen  sulphide  in 
acid  solution.  It  is  sometimes  precipitated,  together  with 
aluminium,  chromium,  titanium,  manganese,  nickel,  cobalt, 
and  zinc,  by  ammonium  sulphide,  and  is  then  separated  from 
the  other  metals.  In  absence  of  the  other  metals  of  this 
group,  it  is  preferable  to  precipitate  iron  and  aluminium 
as  hydroxides  with  ammonia. 

The  separation  of  iron  from  manganese,  etc.,  is  a  matter 
of  some  difficulty.  The  method  usually  recommended  is  the 
basic  acetate  method,  but  the  recently  proposed  "  cupferron  " 
method  appears  to  be  an  improvement. 

Forms  in  -which  Iron  is  precipitated. 

If  not  already  in  the  ferric  state,  the  iron  is  always 
oxidised  before  precipitation. 

Ferric  Hydroxide. — This  is  the  easiest  gravimetric  method 
for  the  determination  of  iron,  but  is  of  limited  applicability. 
For  details,  see  p.  127. 

Basic  Ferric  Acetate. — This  method  provides  a  separation 
from  manganese,  chromium,  zinc,  nickel,  and  cobalt,  but 
aluminium  is  precipitated  with  the  iron.  For  details,  see 
under  Aluminium,  p.  165. 

Ferric  Salt  of  Nitrosophenyl  Hydroxylamine  ("  Cup- 
ferron"  Method). — Iron  is  precipitated  by  "  cupferron  "  from 
strongly  acid  solutions.  This  method  provides  a  complete 
separation  of  iron  from  all  metals  other  than  the  silver  and 
copper  groups,  and  is  particularly  useful  for  the  separation 
of  iron  from  aluminium,  manganese,  and  chromium, 


186          SYSTEMATIC  QUANTITATIVE  ANALYSIS 


Determination  of  Iron  by  the  "Cupferron"  Method. 

OUTLINE  OF  METHOD. — The  iron  (in  the  ferric  state)  is  precipitated 
from  a  strongly  acid  solution  by  "cupferron,"  and  the  precipitate 
is  converted  into  ferric  oxide  by  ignition. 

Cupferron  is  the  ammonium  salt  of  nitrosophenyl  hydrox- 
ylamine,  C6H5(NO)ONH4.  (For  the  method  of  preparation  of 
cupferron,  see  Appendix.)  In  neutral  solution,  it  precipitates 
most  of  the  heavy  metals  as  insoluble  salts.  From  strongly 
acid  solutions,  however,  only  the  cupric  and  ferric  compounds 
are  precipitated.  Copper  is  readily  removed  from  the  solu- 
tion, and  the  method  is  therefore  useful  for  the  separation 
of  iron  from  manganese,  aluminium,  etc.  The  separation  is 
remarkably  complete,  but  in  cases  where  large  amounts  of 
aluminium,  manganese,  or  chromium  are  present,  a  second 
precipitation  is  advisable. 

The  precipitate  of  the  ferric  salt  is  rather  bulky,  and  the 
amount  of  material  taken  should  therefore  be  such  as  to 
yield  about  o- 1  gram  of  ferric  oxide.  The  solubility  of  the 
precipitate  appears  to  be  negligible,  even  in  4N  hydrochloric 
acid. 

Procedure. — To  the  solution  add  20  c.c.  of  concentrated 
hydrochloric  acid,  and  dilute  to  100  c.c.  Dissolve  about  3 
grams  of  cupferron  in  50  c.c.  of  cold  water,  and  add  it 
slowly  and  with  constant  stirring  to  the  ferric  solution.  A 
brownish-red  precipitate,  which  is  partly  crystalline  and 
partly  amorphous,  separates.  Stir  well,  but  do  not  heat  the 
solution. 

Filter  with  suction.  If  the  precipitate  adheres  tenaciously 
to  the  beaker,  dissolve  it  in  a  little  ether  and  then  remove 
the  ether  by  addition  of  a  little  boiling  water. 

Wash  with  cold  water  until  almost  free  from  chloride, 
then  twice  with  dilute  ammonia,  and  finally  twice  with  water. 
Place  the  wet  paper  and  precipitate  in  a  porcelain  crucible, 
and  heat  gently  until  no  more  inflammable  gases  are  given 
off!  Ignite  strongly,  and  weigh  the  ferric  oxide  obtained. 


LEAD  187 

LEAD. 

In  the  analysis  of  a  mixture,  lead  is  usually  determined  as 
sulphate,  though  the  chromate  method  is  preferable  if  it  can 
be  used.  The  separation  of  lead  from  calcium  by  precipita- 
tion as  sulphide  is  described  in  connection  with  the  analysis 
of  glass  (p.  236). 

A  colorimetric  method  for  the  determination  of  traces  of 
lead  is  described  on  p.  161. 
/ 

Forms  in  which  Lead  is  precipitated. 

Lead  Sulphate. — This  method  provides  a  separation  from 
all  metals  except  barium,  strontium,  calcium,  and  mercury. 

Lead  Chromate. — This  method  is  more  accurate  than  the 
sulphate  method,  but  is  limited  in  applicability  on  account  of 
the  general  insolubility  of  chromates. 

Lead  Peroxide  (Electrolytic). —  Lead  may  be  separated 
from  almost  all  other  metals  by  this  method.  For  details, 
seep.  153. 

Hydrated  Lead  Peroxide. — This  method  is  only  used  in 
special  cases,  as  in  the  analysis  of  galena  (see  p.  244). 

Determination  of  Lead  as  Sulphate. 

OUTLINE  OF  METHOD. — The  solution  is  evaporated  with  concentrated 
sulphuric  acid  until  all  hydrochloric  or  nitric  acid  is  expelled.  After 
dilution,  the  lead  sulphate  is  filtered,  washed  with  alcohol,  dried, 
and  weighed  as  PbSO4. 

Lead  Sulphate  is  a  heavy,  white  powder  which  is  sparingly 
soluble  in  water  (i  litre  of  water  dissolves  42  mgrms.  of 
PbSO4  at  1 8°).  It  is  less  soluble  in  dilute  sulphuric  acid,  but 
with  increasing  concentration  of  sulphuric  acid  the  solubility 
again  increases.  It  is  readily  soluble  in  concentrated  hydro- 
chloric acid,  and  somewhat  less  so  in  nitric  acid.  It  is  soluble 
in  solutions  of  almost  all  ammonium  salts  and  in  solutions 
of  alkali  hydroxides,  but  is  almost  insoluble  in  alcohol.  It 
may  be  heated  to  a  bright  red  heat  without  decomposition 
if  reducing  gases  are  excluded  from  the  crucible.  At  a  red 
heat  it  is  readily  reduced  by  carbonaceous  matter,  with  loss 
of  lead  by  volatilisation. 


188          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Procedure. — To  the  lead  solution,  add  5  c.c.  of  con- 
centrated sulphuric  acid,  and  evaporate  in  a  porcelain  basin 
over  a  Rose  burner  until  dense  white  fumes  are  evolved. 
Cool,  dilute  to  about  50  c.c.  with  cold  water,  and  stir.  The 
precipitate  may  be  filtered  more  readily  if  it  is  kept  for  a  few 
hours  before  filtration. 

Filter  through  a  Gooch  crucible,  wash  two  or  three  times 
with  water  acidified  with  sulphuric  acid,  and  then  wash  with 
alcohol  until  free  from  acid. 

If  the  original  solution  contained  other  metals,  the  pre- 
cipitate must  be  washed  six  to  eight  times  with  the 
minimum  quantity  of  a  mixture  of  dilute  sulphuric  acid 
and  water,  before  using  alcohol ;  the  alcohol  washings  are 
then  rejected. 

Dry  at  100°,  place  the  Gooch  crucible  inside  a  nickel 
crucible,  and  heat  strongly  until  of  constant  weight. 

Determination  of  Lead  as  Chroma te. 

OUTLINE  OF  METHOD. — The  lead  is  precipitated  as  chromate  by  the 
addition  of  potassium  chromate  (or  dichromate).  The  precipitate  is 
collected  in  a  Gooch  crucible,  dried  at  120°,  and  weighed  as  PbCrO4. 

Lead  Chromate  is  almost  insoluble  in  water  and  in  acetic 
acid  (i  litre  of  water  dissolves  02  mgrm.  at  18°),  slightly 
soluble  in  nitric  acid,  and  readily  soluble  in  alkali  solutions. 
It  may  be  dried  completely  at  100°,  but  loses  oxygen  if 
heated  to  its  melting  point. 

Procedure. — If  the  solution  is  neutral  or  alkaline,  add 
acetic  acid  until  it  is  distinctly  acid.  If  the  solution  contains 
nitric  acid,  add  sufficient  sodium  acetate  to  replace  the  nitric 
acid  by  acetic  acid.  To  the  hot  solution  add  potassium 
chromate  in  slight  excess  (until  the  supernatant  liquid  is 
slightly  coloured),  and  keep  the  solution  warm  for  a  few 
minutes.  Cool,  filter  through  a  Gooch  crucible,  wash 
thoroughly  with  cold  water,  and  dry  at  120°. 

MAGNESIUM. 

On  account  of  their  alkaline  character,  magnesium 
carbonate  and  hydroxide  may  be  determined  volumetrically, 


MANGANESE  189 

but  there  is  no  volumetric  process  applicable  to  magnesium 
salts  in  general. 

In  the  analysis  of  a  mixture,  magnesium  is  always  deter- 
mined after  removal  of  the  copper,  iron,  zinc,  and  calcium 
groups.  A  typical  example  of  the  separation  of  magnesium 
from  calcium  and  other  metals  is  described  on  p.  228. 

The  only  gravimetric  method  for  the  determination  of 
magnesium  has  already  been  described  (p.  135). 

MANGANESE. 

In  gravimetric  analysis,  manganese  is  always  determined 
after  removal  of  the  metals  precipitated  in  acid  solution  by 
hydrogen  sulphide.  Hillebrand,  in  his  Analysis  of  Silicate 
and  Carbonate  Rocks,  states  that  "  the  gravimetric  determina- 
tion of  manganese  in  small  amount  seems  to  be  more  of  a 
stumbling  block  to  the  average  chemist  than  that  of  almost 
any  other  of  the  frequently  occurring  elements  in  mineral 
analysis.  This  is  due  almost  always  to  incomplete  prior  pre- 
cipitation of  elements  which  later  suffer! co-precipitation  with 
the  manganese." 

A  colorimetric  method  for  the  determination  of  traces  of 
manganese  is  described  on  p.  162. 

Forms  in  which  Manganese  is  precipitated. 

Manganous  Carbonate. — This  method  is  applicable  in  the 
absence  of  other  metals  which  form  insoluble  carbonates,  and 
only  the  alkalis  and  ammonium  salts  may  be  present. 

Manganese  Dioxide  (Hydrated). — Precipitation  in  this 
form  by  means  of  ammonium  persulphate  provides  a  method 
of  separation  from  chromium.  If  more  than  traces  of  zinc, 
nickel,  and  cobalt  are  present,  a  second  precipitation  is 
necessary  for  complete  separation. 

Manganous  Sulphide. — In  this  form,  manganese  is  pre- 
cipitated by  means  of  ammonium  sulphide  along  with  zinc, 
nickel,  and  cobalt  (and  possible  traces  of  copper).  The 
sulphides  of  manganese  and  zinc  are  then  separated  from 
the  others  by  means  of  normal  hydrochloric  acid  containing 
hydrogen  sulphide. 


190          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Determination  of  Manganese  as  Carbonate. 

OUTLINE  OF  METHOD. — The  manganese  is  precipitated  as  manganous 
carbonate  by  means  of  ammonium  carbonate.  By  ignition,  first  in 
air  and  then  in  a  current  of  carbon  dioxide,  the  precipitate  is 
converted  into  Mn3O4,  which  is  weighed  ;  or  it  may  be  converted 
into  and  weighed  in  one  or  other  of  the  forms  mentioned 
below. 

Manganous  Carbonate  is  a  buff-coloured  powder  which 
darkens  somewhat  on  exposure  to  air  and  is  sometimes 
difficult  to  filter.  It  is  practically  insoluble  in  water  and  in 
solutions  of  ammonium  salts.  It  dissolves  in  acids  and  is 
slightly  soluble  in  solutions  of  alkali  carbonates. 

At  a  high  temperature,  with  access  of  air,  manganous 
carbonate  is  converted  mainly  into  Mn3O4,  together  with  traces 
of  Mn2O3  and  MnO2,  the  actual  composition  of  the  residue 
depending  on  the  manner  of  ignition.  This  mixture  of 
oxides  may  be  converted  quantitatively  into — 

(1)  MnO  (green),  by  heating  at  a  high  temperature  in  a 

current  of  hydrogen  ; 

(2)  Mn3O4  (brown),  by  heating  to  a  high  temperature  in  a 

current  of  carbon  dioxide ; 

(3)  Mn2O3  (black),  by  heating  to  low  redness  in  a  current 

of  oxygen ; 

(4)  MnS  (green),  by  heating  with  sulphur  in  hydrogen ; 

(5)  MnSO4  (white),  by  adding  sulphuric  acid  and  heating 

to  low  redness. 

Procedure. — If  the  manganese  is  present  as  permanganate, 
it  must  first  be  reduced  to  a  manganous  salt  by  means  of 
sulphur  dioxide  and  the  excess  of  sulphur  dioxide  expelled 
by  boiling. 

Neutralise  the  solution  with  ammonia,  add  10  grams 
of  ammonium  chloride  and  a  slight  excess  of  ammonium 
carbonate.  Allow  the  beaker  to  remain  on  a  gently  heated 
steam-bath  until  the  precipitate  has  settled  completely. 
Filter,  and  wash  the  precipitate  with  hot  water.  Incinerate 
the  filter  together  with  the  precipitate  in  a  Rose  crucible, 
and  ignite  the  precipitate  in  the  open  crucible  for  ten  minutes 
with  a  Meker  or  Teclu  burner.  Then  pass  a  slow  current  of 
carbon  dioxide  into  the  crucible,  and  heat  again  to  a  high 


MANGANESE  191 

temperature  for  ten  minutes.  Cool  in  an  atmosphere  of 
carbon  dioxide,  and  weigh  as  Mn3O4.  Repeat  until  constant 
weight  is  attained. 

In  order  to  check  the  result,  heat  the  oxide  with  a  Meker 
burner  in  a  rapid  current  of  hydrogen  for  five  minutes,  and 
weigh  as  MnO.  Repeat  until  the  weight  is  constant. 

From  the  weight  of  Mn3O4  or  MnO,  calculate  the  per- 
centage of  manganese  in  the  substance  taken  for  analysis. 

Determination  of  Manganese  as  Dioxide. 

OUTLINE  OF  METHOD. — The  manganese  is  precipitated  as  hydrated 
manganese  dioxide  by  boiling  an  acidified  solution  with  ammonium 
persulphate.  The  dioxide  is  converted  by  ignition  into  a  lower 
oxide,  which  is  weighed. 

Hydrated  Manganese  Dioxide  is  a  brownish-black  pre- 
cipitate which  is  insoluble  in  water,  alkalis,  dilute  sulphuric 
or  nitric  acid.  It  is  somewhat  soluble  in  concentrated  nitric 
acid,  readily  soluble  in  hydrochloric  acid  with  evolution  of 
chlorine,  and  in  concentrated  sulphuric  acid  with  evolution 
of  oxygen.  It  is  insoluble  in  solutions  of  ammonium  salts. 
If  precipitated  in  alkaline  solution,  it  occludes  alkali  which 
cannot  be  removed  by  washing.  The  dioxide  itself  is 
unsuitable  for  weighing  and  is  always  converted  into  one  of 
the  lower  oxides  by  ignition,  as  already  described. 

Procedure. — Dilute  the  solution,  which  should  be  slightly 
acid  with  sulphuric  acid,  but  must  contain  no  other  acid,  to 
200  c.c.  To  the  cold  solution,  add  2  grams  of  ammonium 
persulphate  dissolved  in  about  50  c.c.  of  water,  and  heat 
quickly  until  boiling.  Boil  for  two  minutes,  filter  at  once 
without  cooling,  and  wash  the  precipitate  thoroughly  with 
hot  water.  The  filtrate  should  be  colourless  ;  set  it  aside 
on  the  steam-bath  for  an  hour.  No  further  precipitate 
should  form,  but  if  there  is  any,  filter,  and  add  it  to  the 
main  precipitate. 

Incinerate  the  filter  together  with  the  precipitate  in  a 
Rose  crucible,  and  convert  into  a  lower  oxide  suitable  for 
weighing,  as  already  described. 

The  above  method  affords  a  complete  separation  from 
chromium.  If  other  metals,  such  as  iron,  zinc,  nickel,  and 
cobalt  are  present,  a  second  precipitation  is  necessary,  and 


192          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

even  then  the  separation  may  not  be  complete.  Dissolve 
the  precipitate,  after  thorough  washing,  in  as  little  hot 
concentrated  hydrochloric  acid  as  possible.  Add  I  c.c.  of 
concentrated  sulphuric  acid,  evaporate  until  the  hydrochloric 
acid  is  expelled,  cool,  and  dilute  with  water.  Reprecipitate 
as  before. 

Notes. — (i)  The  conditions  for  the  precipitation  must  be 
closely  adhered  to,  otherwise  precipitation  is  incomplete. 

(2)  The  purity  of  the  ammonium  persulphate  should  be 
tested.  There  should  be  no  precipitation  of  alumina  when 
a  solution  is  boiled  with  ammonia.  No  weighable  residue 
should  be  left  after  ignition  of  2  grams  in  a  platinum 
crucible. 

MERCURY. 

Mercury  may  be  determined  volumetrically  by  the  methods 
given  on  pp.  56  and  104.  The  thiocyanate  titration  is  con- 
venient and  accurate. 

Forms  in  which  Mercury  is  precipitated. 

Mercuric  Sulphide. — The  mercury  must  all  be  present 
as  a  mercuric  salt.  This  method  is  recommended  when 
applicable,  *>.,  in  absence  of  those  metals  which  are  pre- 
cipitated in  a  similar  manner. 

Metallic  Mercury. — This  method  is  useful  for  separating 
mercury  from  all  other  metals.  It  is  applicable  to  mercury 
in  any  form  of  combination. 

Determination  of  Mercury  as  Sulphide. 

OUTLINE  OF  METHOD. — The  mercury  is  precipitated  with  hydrogen 
sulphide  in  acid  solution,  and,  after  removal  of  any  free  sulphur  by 
extraction  with  carbon  disulphide,  is  weighed  as  HgS. 

Procedure. — The  mercury,  if  not  already  in  the  mercuric 
state,  must  be  oxidised  by  boiling  with  concentrated  nitric 
acid.  The  presence  of  nitric  acid  in  the  solution  is  objection- 
able, as  it  gives  sulphur  with  hydrogen  sulphide.  It  is  not 
permissible  to  remove  it,  however,  by  evaporation  with 
hydrochloric  acid,  as  mercuric  chloride  would  be  volatilised 
and  lost  in  the  process. 


MERCURY 


193 


FIG.  52. 


Saturate  the  cold  solution  with  hydrogen  sulphide,  filter 
through  a  Gooch  crucible,  wash  with  cold  water   and  then 
two  or  three  times  with  alcohol.     The  sulphur 
in  the  precipitate  is  removed  by  extraction  with 
carbon  disulphide.      Carbon  disulphide  usually 
contains  some  dissolved  sulphur,  and  the  follow- 
ing method  of  extraction    is    therefore  recom- 
mended : — Place  the  crucible  on  a  glass  triangle 
within    a    beaker  which  contains    some  carbon 
disulphide.     Cover  the  beaker  with  a  flask  con- 
taining cold  water  (see  Fig.  52),  and  heat  the 
beaker  on  the  steam-bath.     Within  an  hour  all 
the  sulphur  will  be  extracted.     Wash  twice  with 
alcohol  to  remove  carbon  disulphide,  and  dry  at  100°  to  1 10°. 
Weigh  the  HgS. 

Determination  of  Mercury  as  Metal. 

OUTLINE  OF  METHOD. — The  dry  substance  is  heated  with  a  mixture  of 
quicklime,  iron  filings,  and  lead  chromate.  The  mercury  which  is 
driven  off  is  collected  and  weighed. 

Procedure. — A  convenient  apparatus  is  that  devised  by 
Penfield  for  the  determination  of  water  in  minerals.  Close  a 
piece  of  glass  tubing  (about  20  cm.  in  length  and  5  mm.  in 
diameter)  at  one  end,  and  blow  bulbs  at  A  and  B,  Fig.  53. 

IT  B 


FIG.  53- 

Clean,  dry,  and  weigh  the  tube.  By  means  of  the  long 
funnel  C,  introduce  0-5  to  i-o  gram  of  the  powdered  material 
into  the  bulb  A.  Weigh  the  tube  and  contents. 

Add  some  iron  filings  by  means  of  the  funnel  C,  from 
which  any  adhering  trace  of  the  substance  has  meanwhile 
been  removed.  Mix  the  substance  and  filings  thoroughly  by 
rotating  the  tube.  Then  add  a  mixture  of  one  part  quicklime, 
one  part  powdered  (fused)  lead  chromate,  and  two  parts 

N 


11)4 


SYSTEMATIC  QUANTITATIVE  ANALYSIS 


iron  filings,  until  about  8  cm.  of  the  tube  has  been  filled. 
Insert  a  small  plug  of  asbestos,  E,  so  that,  after  tapping  the 
tube,  only  a  very  shallow  air  -  channel  remains  over  the 
mixture.  Draw  out  the  open  end  D  into  a  fine  capillary,  as 
shown  at  F,  Fig.  54. 

Place  the  prepared  tube  in  an  iron  tube  which  can  be 
heated  by  a  flat-flame  burner.  (A  piece  of  iron  gas-pipe 
about  15  cm.  by  1-5  cm.,  closed  at  the  ends  with  plugs  of 
asbestos,  may  be  used.)  Wrap  the  bulb  A  in  some  asbestos 
paper  to  prevent  it  coming  into  direct  contact  with  the  iron 
tube ;  if  this  precaution  is  omitted,  the  bulb  becomes  hot 
before  the  narrow  portion  of  the  tube,  and  the  mercuric 
salt  is  partly  volatilised  without  decomposition.  Place  an 
asbestos  shield  at  G,  and  cover  the  bulb  B  with  wet  filter 
paper. 


0 


FIG.  54. 

Heat  the  iron  tube,  at  first  with  a  small  flame  and  at  the 
end  nearer  G  only.  Gradually  increase  the  flame  and  move 
it  until  the  whole  iron  tube  is  heated  to  a  low  red  heat.  The 
apparatus  must  be  almost  horizontal,  but  with  the  end  F 
slightly  lower  than  the  closed  end,  so  that,  on  gently  tapping 
the  tube,  any  mercury  which  has  condensed  forms  a  globule 
and  runs  into  the  bulb  B. 

When  all  the  mercury  has  distilled  (usually  within  one 
hour),  draw  out  the  glass  tube  until  the  plug  E  is  exposed. 
At  the  same  time  move  the  burner  forward,  so  that  the  flame 
plays  directly  on  the  glass  tube.  As  soon  as  the  glass  tube 
becomes  red  hot,  draw  it  off  about  midway  between  the  plug 
E  and  the  bulb  B.  The  mercury  is  thus  obtained  in  a  tube, 
as  shown  at  H  in  Fig.  54. 


NICKEL  195 

To  remove  water,  draw  a  current  of  dry  air  through  the 
tube  until  the  weight  is  constant.  Shake  out  the  main 
portion  of  the  mercury,  and  remove  the  remainder  by  blow- 
ing air  through  the  gently  heated  tube.  (Caution. — Mercury 
vapour  is  dangerous.)  Weigh  the  empty  tube  after  cooling. 

NICKEL. 

Nickel  is  determined  after  removal  of  metals  which 
are  precipitated  by  hydrogen  sulphide  in  acid  solution. 
Although  a  pure  nickel  solution  yields  no  precipitate  with 
hydrogen  sulphide  in  acid  solution,  some  nickel  is  co- 
precipitated  with  the  sulphides  of  the  copper  and  arsenic 
group  unless  the  solution  is  strongly  acid. 

Forms  in  which  Nickel  is  precipitated. 

Nickel  Sulphide.  —  Ammonium  sulphide  precipitates 
nickel,  together  with  the  other  metals  of  the  iron  and  zinc 
group,  and  this  method  serves  to  separate  nickel,  iron,  zinc, 
etc.,  from  the  alkalis  and  alkaline  earths.  Nickel  sulphide 
is  not  precipitated  from  acid  solutions,  but  the  precipitated 
sulphide  dissolves  so  slowly  in  acids  that  it  may  be  regarded 
(for  analytical  purposes)  as  insoluble  in  cold  dilute  acids. 

Nickel  Peroxide. — This  is  the  form  in  which  nickel  is 
usually  precipitated  if  no  other  metals  are  present. 

Metallic  Nickel  (Electrolytic). — This  is  a  useful  and 
accurate  method  for  the  determination  of  nickel.  The 
method  can  be  adapted  to  provide  a  separation  of  nickel 
from  copper  and  other  metals  (see  p.  153). 

Determination  of  Nickel  as  Oxide. 

OUTLINE  OF  METHOD. — The  nickel  is  precipitated  as  nickel  peroxide 
by  means  of  bromine  and  sodium  hydroxide.  The  precipitate  is 
washed,  dried,  and  ignited.  It  is  then  ignited  in  an  atmosphere  of 
hydrogen  and  the  metallic  nickel  weighed. 

Nickel  Peroxide,  Ni2O3,  when  freshly  precipitated,  is  a 
brownish-black  substance  which  becomes  darker  on  keeping. 
It  is  insoluble  in  hot  or  cold  water  or  in  alkaline  solutions. 
The  peroxide,  unlike  nickelous  hydroxide,  does  not  occlude 
alkali  salts.  On  ignition,  it  is  converted  almost  entirely  into 
nickel  oxide,  NiO,  but  the  composition  of  the  residue 


196          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

varies  considerably  with  the  manner  of  ignition.  Nickel 
oxide  is  completely  reduced  to  metal  by  ignition  in  hydrogen. 
The  metallic  nickel  may  be  contaminated  with  silica  dis- 
solved from  the  glass  or  porcelain  vessel  by  the  sodium 
hydroxide. 

Procedure. — Dilute  the  nickel  solution,  contained  in  a 
porcelain  beaker  or  basin,  to  about  100  c.c.,  and  add  about 
I  c.c.  of  bromine.  Warm  the  solution,  and  precipitate  the 
nickel  with  pure  sodium  hydroxide  solution,1  avoiding 
excess.  Heat  until  boiling.  Any  signs  of  greenish,  nickelous 
hydroxide  indicate  that  insufficient  bromine  has  been 
added. 

Filter  off  the  precipitate,  wash  with  hot  water  (as  far  as 
possible  by  decantation),  and  dry.  Incinerate  the  filter 
paper  apart  from  the  precipitate  in  a  Rose  crucible,  add  the 
precipitate,  and  ignite  strongly  for  a  few  minutes.  Allow 
the  crucible  to  cool ;  pass  a  current  of  pure  dry  hydrogen, 
and  heat  the  crucible  to  dull  redness  for  about  twenty 
minutes.  Continue  the  current  of  hydrogen  until  the 
crucible  is  cold,  in  order  to  prevent  reoxidation.  Repeat 
the  ignition  in  hydrogen  until  the  weight  has  become 
constant.  The  metallic  nickel  obtained  in  this  way  contains 
traces  of  silica.  If  the  precipitation  has  been  performed  in 
a  porcelain  vessel  without  using  a  large  excess  of  alkali, 
the  amount  of  silica  will  be  very  small ;  but  in  all  cases 
the  silica  should  be  determined  and  a  correction  applied. 
Dissolve  the  nickel  in  nitric  acid,  and  evaporate  to  dryness. 
Moisten  the  residue  with  a  few  drops  of  concentrated  acid, 
dilute  with  water,  filter  through  a  small  filter  paper,  and 
wash  until  the  residue  is  colourless.  Incinerate  the  paper 
with  the  residue  of  silica,  ignite,  and  weigh.  Subtract  the 
weight  of  silica  from  that  of  the  impure  nickel. 

PHOSPHATE. 
Forms  in  -which  Phosphate  is  precipitated. 

Ammonium  Phospho-molybdate. — The  chief  value  of  this 
method  is  that  it  is  available  in  presence  of  the  alkaline 
earths,  aluminium,  and  iron. 

1  Prepared  from  metallic  sodium  (see  note  on  p.  140). 


PHOSPHATE  197 

/ 

Magnesium  Ammonium  Phosphate. — This  method  is 
not  available  in  presence  of  metallic  radicals  other  than  the 
alkalis. 

Determination  of  Phosphate  by  the  Molybdate  Method. 

OUTLINE  OF  METHOD. — The  phosphate  is  precipitated  in  presence 
of  nitric  acid  by  ammonium  molybdate.  The  precipitate  is  collected 
in  a  Gooch  crucible,  ignited,  and  weighed  as  phospho-molybdic 
anhydride,  24MoO3,  P2O5. 

A  m  1 /ionium  Phospho- molybdate — 

(NH4)3P04,  i2Mo03,  *H20, 

is  a  bright  yellow,  crystalline  substance,  insoluble  in  dilute 
nitric  and  sulphuric  acids,  but  somewhat  soluble  in  hydro- 
chloric acid.  It  is  readily  soluble  in  ammonia,  and  soluble 
also  to  some  extent  in  solutions  containing  chlorides  or 
ammonium  salts  (except  the  nitrate). 

It  may  be  weighed  as  the  anhydrous  salt  after  drying  at 
100°,  but  it  is  better  to  convert  it  into  phospho-molybdic 
anhydride,  24MoO3,P2O5,  by  gentle  ignition.  The  anhydride 
must  not  be  heated  too  strongly,  otherwise  phosphoric 
anhydride  will  be  volatilised  and  lost. 

On  account  of  the  small  proportion  of  phosphorus  in  the 
final  precipitate,  an  amount  of  substance  containing  about 
10  mgrms.  of  P2O5  is  ample  for  an  analysis. 

The  following  solutions  are  required  : — 

(1)  Ammonium  Molybdate. — Dissolve  100  grams  in    500 
c.c.  of  hot  water,  cool  the  solution  and  pour  it  into  500  c.c. 
of  concentrated   nitric  acid.     After  a  day  or  two,  filter  the 
solution  and  keep  in  a  well-stoppered  bottle. 

(2)  A  Washing  Solution  containing  50  grams  of  ammonium 
nitrate  and  40  c.c.  of  concentrated  nitric  acid  per  litre  of 
water. 

Preparation  of  a  Solution  for  Analysis. — In  most  cases, 
all  the  phosphorus  can  be  obtained  in  solution  by  boiling 
the  substance  with  concentrated  nitric  acid.  For  complex 
rocks  containing  silicates,  however,  fusion  with  sodium 
carbonate  is  necessary,  and  the  silica  must  be  removed  by 
evaporation  to  dryness  in  presence  of  nitric  acid. 


198          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

In  all  cases,  it  is  necessary  that  the  phosphorus  should  be 
in  the  form  of  ortho-phosphate ;  meta-  or  pyro-phosphates 
must  therefore  be  boiled  with  dilute  nitric  acid  for  a  few 
minutes. 

Procedure. — Acidify  the  solution  with  nitric  acid  and 
evaporate,  if  necessary,  to  about  80  c.c.  Add  about  8  grams 
of  solid  ammonium  nitrate  and  heat  until  nearly  boiling. 
Heat  50  c.c.  of  ammonium  molybdate  solution  until  boiling, 
and  add  it  slowly  and  with  constant  stirring  to  the  hot 
phosphate  solution.  Take  care  not  to  touch  the  side  of  the 
beaker  with  the  stirring-rod.  Stir  for  a  few  minutes.  After 
about  twenty  minutes,  filter  through  a  Gooch  crucible  and 
wash  with  the  washing  solution  mentioned  above,  using 
about  eight  portions  of  washing  solution. 

Place  the  Gooch  crucible  inside  a  platinum  crucible  which 
is  just  large  enough  to  hold  it,  and  heat  gently  until  there  is 
no  further  evolution  of  ammonia.  Ignite  to  dull  redness 
for  about  ten  minutes,  and  weigh  the  greenish-black  phospho- 
molybdic  anhydride  obtained. 

Determination  of  Phosphate  as  Magnesium 
Py  r  ophospha  t  e . 

(Precipitation  as  Magnesium  Ammonium  Phosphate^} 

OUTLINE  OF  METHOD. — The  phosphate  is  precipitated  by  addition  of 
"magnesia  mixture"  as  magnesium  ammonium  phosphate,  which  is 
converted  by  ignition  into  magnesium  pyrophosphate,  Mg2P2O7. 

Magnesium  Ammonium  Phosphate,  MgNH4PO4. —  The 
properties  of  this  precipitate,  and  the  conditions  under  which 
it  may  be  obtained  pure,  are  described  on  p.  135.  The 
precipitate  has  the  composition  corresponding  to  the  normal 
salt,  MgNH4PO4,  only  when  these  conditions  are  rigidly 
adhered  to. 

Magnesium  Pyrophosphate,  Mg2P2O7,  has  already  been 
described  (p.  136). 

Procedure. — The  phosphate  must  all  be  present  as  ortho- 
phosphate  ;  if  any  pyro-  or  meta-salt  is  present,  the  solution 
must  be  [boiled  for  some  time  with  dilute  hydrochloric 
or  nitric  acid. 


POTASSIUM  AND  SODIUM  199 

Add  ammonia  until  the  solution  is  slightly  alkaline  and 
evaporate,  if  necessary,  to  100  c.c.  For  every  02  gram  of 
P2O5  present  (or  believed  to  be  present),  add  15  c.c.  of 
magnesia  mixture  (p.  168).  The  magnesia  mixture  must  be 
added  quickly  and  with  constant  stirring ;  if  added  slowly, 
the  precipitate  will  not  be  of  the  correct  composition. 

The  precipitate  should  separate  slowly  and  appear 
crystalline.  After  about  twenty  minutes,  add  10  c.c.  of 
concentrated  ammonia,  and  keep  for  at  least  four  hours 
before  filtration.  Filter,  using  slight  suction,  and  wash  with 
dilute  ammonia  until  the  washings  are  free  from  chloride. 
Avoid  over-washing,  as  the  precipitate  dissolves  to  a  slight 
extent  even  in  dilute  ammonia. 

Dry,  ignite,  and  weigh  as  Mg2P2O7.  For  details  of  this 
part  of  the  procedure,  see  p.  136. 


POTASSIUM   AND   SODIUM. 

Potassium  and  sodium  usually  occur  together.  Potassium 
may  be  determined  without  determining  sodium,  but  the 
determination  of  sodium  involves  that  of  potassium.  In 
order  to  determine  both  potassium  and  sodium,  all  other 
metals  must  be  removed  and  the  residual  solution  evaporated 
to  dryness.  The  residue  is  completely  converted  into 
chloride  and  weighed.  The  amount  of  potassium  in  the 
mixed  salts  is  then  ascertained  by  one  or  other  of  the 
methods  given  below,  and  the  sodium  found  by  difference. 
For  details,  see  p.  204. 

The  determination  of  sodium  and  potassium  in  an 
insoluble  silicate  is  described  on  p.  234. 

Forms  in  which  Potassium  is  Precipitated. 

Potassium  Ohloroplatinate. — Ammonium  salts  and  all 
metals  other  than  sodium  and  potassium  must  be  removed. 
Sulphate  and  phosphate  must  also  be  removed. 

Potassium  Perchlorate. — The  special  advantages  of  this 
method  are  that  the  use  of  expensive  platinum  salts  is 
avoided,  and  that  it  is  applicable  in  presence  of  phosphate 
and  of  most  metals.  The  only  common  acidic  radical  that 


200          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

must  be  removed  is  sulphate.  Ammonium  salts,  if  present 
in  quantity,  must  be  removed,  but  small  amounts  do  not 
interfere  with  the  method. 


Preparation  of  a  Solution  for  Analysis. 

(1)  In    the    Complete    Analysis   of  a   Mixture^   all   other 
metals  are  removed  prior  to  the  determination  of  sodium 
and    potassium.     If  any  sulphate  or   phosphate   is   present, 
add  barium  hydroxide  solution  in  slight  excess  and,  without 
filtration,  evaporate  to  about  50  c.c.     Add  a  few  drops  of 
freshly  prepared  ammonium  carbonate  solution  in  order  to 
precipitate  the  barium  salt,  filter  through  a  small  filter  paper, 
and   wash   with   hot   water.      Determine    the    sodium    and 
potassium  in  the  filtrate. 

(2)  If  only  Potassium  is  to  be  determined^  extract  a  weighed 
sample  of  the  original  material  with  hot,  dilute  hydrochloric 
acid,  and   filter   from    any   insoluble   matter.     (With    many 
minerals,  particularly  silicates,  it  is  not  possible  to  bring  all 
the  potassium  into  solution  as  a  soluble  salt  by  treatment 
with  hydrochloric  acid.     The  procedure  for  a  case  of  this  kind 
is  described  on  p.  234.)     Evaporate  the  solution    of  mixed 
chlorides   to  dryness  in  a   porcelain  basin,  and  heat  on   a 
sand-bath  to  barely-visible  redness  for  about  fifteen  minutes, 
in  order  to  convert  iron,  aluminium,  etc.,  into  insoluble  basic 
salts.     The  duration  of  the  ignition  should  be  such  that,  on 
extracting  the  residue  with  water,  a  colourless  solution,  free 
from  iron,  is  obtained. 

When  sulphate  is  present,  it  must  be  removed  after  the 
evaporation  to  dryness  by  adding  a  slight  excess  of  barium 
hydroxide  solution,  and  completing  the  evaporation  and 
ignition  as  described  above. 

Extract  the  soluble  alkali  salts  by  repeated  treatment 
with  boiling  water,  breaking  up  the  insoluble  residue  as 
much  as  possible  with  a  glass  rod.  Filter  into  a  glass 
evaporating  basin,  and  determine  the  potassium  by  the 
perchlorate  method, 


POTASSIUM  201 


Determination  of  Potassium  as  Chloroplatinate. 

OUTLINE  OF  METHOD. — After  removal  of  sulphate,  phosphate, 
ammonium  salts,  and  all  metals  other  than  sodium  and  potassium, 
hydrochloroplatinic  acid  is  added,  and  the  solution  is  evaporated 
to  dryness.  The  residue  is  extracted  with  methyl  alcohol,  which 
dissolves  the  sodium  chloroplatinate.  The  insoluble  residue  of 
potassium  chloroplatinate  is  dried  and  weighed. 

Potassium  Chloroplatinate  is  a  golden-yellow  crystalline 
salt  which  has  the  composition  indicated  by  the  formula 
K2PtCl6.  It  is  insoluble  in  ethyl  alcohol  and  in  methyl 
alcohol,  and  is  sparingly  soluble  in  water.  If  the  salt  is 
washed  with  water  or  with  dilute  alcohol,  a  portion  dis- 
solves and  the  remainder  becomes  partially  hydrolysed ;  the 
residual  potassium  chloroplatinate  is  thus  contaminated 
with  potassium  salts  of  other  platinum  acids.  Even  ordinary 
"  absolute  "  alcohol  promotes  this  hydrolytic  change  to  some 
extent.  For  this  reason,  the  precipitate  obtained  in 
quantitative  analysis  is,  after  washing  and  drying,  not 
pure  K2PtClG;  it  is,  however,  of  constant  composition,  if 
the  working  conditions  are  always  the  same.  The  precipitate 
obtained  by  adopting  the  procedure  described  below 
contains  16-04  Per  cent,  of  potassium  (calculated  for 
K2PtCl6,  K  =16-08  per  cent).  . 

As  ammonium  chloroplatinate  is  also  insoluble  in  alcohol, 
it  is  essential  to  remove  all  ammonium  salts  prior  to  the 
precipitation,  and  also  to  guard  against  contamination  of 
the  solution  by  ammonia  from  the  laboratory  atmosphere 
during  the  analysis. 

Sodium  Chloroplatinate  is  an  orange-red  salt  which  is 
soluble  in  absolute  ethyl  alcohol,  somewhat  more  soluble  in 
absolute  methyl  alcohol,  and  readily  soluble  in  water.  In 
order  to  separate  sodium  and  potassium  chloroplatinates, 
it  is  best  to  use  absolute  methyl  alcohol,  as  absolute  ethyl 
alcohol  does  not  dissolve  the  sodium  salt  sufficiently  readily. 
The  use  of  70  per  cent,  ethyl  alcohol  is  sometimes  recom- 
mended, but  this  dilute  alcohol  causes  excessive  hydrolytic 
decomposition  of  the  potassium  salt. 

The  precipitated,  hydrated  salt  (Na2PtCl6,  6H2O)  be- 
comes anhydrous  at  the  temperature  of  the  steam-bath. 


202          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

The  anhydrous  salt  is  much  more  soluble  in  absolute  alcohol 
than  the  hydrated  salt. 

Procedure. — The  solution,  after  removal  of  sulphate, 
phosphate,  and  all  metals  other  than  sodium  and  potassium, 
is  evaporated  to  dryness.  The  sodium  and  potassium  are 
obtained  finally  in  a  platinum  crucible  as  pure  chlorides, 
and  weighed.  Full  details  of  the  procedure  up  to  this  stage 
are  given  on  p.  205.  The  potassium  is  then  determined  as 
chloroplatinate,  as  follows  : — 

On  account  of  the  insolubility  of  sodium  chloride  in 
alcohol,  it  is  necessary  to  convert  both  the  sodium  and 
potassium  chlorides  into  chloroplatinates.  The  amount  of 
hydrochloroplatinic  acid  necessary  to  effect  this  is  found 
by  calculation  from  the  weight  of  the  mixed  chlorides, 
making  the  assumption  that  the  whole  of  the  chloride  is 
sodium  chloride.  As  the  reagent  contains  10  per  cent,  of 
platinum  (see  p.  364),  17  c.c.  is  required  for  I  gram  of  sodium 
chloride. 

To  the  mixed  chlorides  in  the  platinum  crucible,  add 
about  0-3  c.c.  more  than  the  calculated  quantity  of  hydro- 
chloroplatinic acid,  and  evaporate  to  complete  dryness  on 
the  steam-bath. 

Add  about  5  c.c.  of  absolute  methyl  alcohol  to  the  dry 
residue  and  break  up  the  mass  thoroughly  with  a  platinum 
spatula  or  glass  rod.  Decant  the  liquid  through  a  Gooch 
filter  which  has  been  previously  dried  at  160°  and  weighed. 
The  solid  must  be  kept  as  far  as  possible  in  the  platinum 
crucible,  only  the  clear  (supernatant)  liquid  being  poured 
into  the  Gooch  filter.  Repeat  this  treatment  with  successive 
small  quantities  of  methyl  alcohol  (grinding  up  the  precipitate 
each  time  as  thoroughly  as  possible)  until  the  filtrate  is 
colourless,  and  until  no  orange-red  particles  of  the  sodium 
salt  can  be  seen  amongst  the  golden-yellow  potassium 
chloroplatinate.  When  the  washing  is  complete,  bring  the 
precipitate  into  the  Gooch  filter,  drain  off  the  alcohol  by 
means  of  the  filter-pump,  dry  at  160°,  and  weigh. 

The  weight  of  the  dried  precipitate  multiplied  by  0-1604 
gives  the  weight  of  potassium.  The  weight  of  the  dried 
precipitate  multiplied  by  0-3059  gives  the  weight  of  potassium 
chloride. 


POTASSIUM  203 

The  weight  of  sodium  chloride  is  found  by  subtracting 
that  of  the  potassium  chloride  from  the  weight  of  the  mixed 
chlorides. 

Determination  of  Potassium  as  Perchlorate. 

OUTLINE  OF  METHOD. — The  solution  of  mixed  chlorides  is  evaporated 
almost  to  dryness  with  perchloric  acid.  All  salts  other  than 
potassium  perchlorate  are  then  extracted  with  alcohol  and  the 
insoluble  residue  is  weighed  as  KC1O4. 

Potassium  Perchlorate,  KC1O4,  is  a  sparingly  soluble 
crystalline  salt.  One  litre  of  water  at  20°  dissolves  16-7 
grams,  I  litre  of  97  per  cent,  alcohol  dissolves  0-16  gram, 
whilst  I  litre  of  97  per  cent,  alcohol  containing  02  per  cent, 
of  perchloric  acid  dissolves  about  0-05  gram.  The  solubility 
in  all  solvents  increases  rapidly  as  the  temperature  is  raised. 
The  salt  may  be  dried  without  decomposition  above  100°  and 
below  200°. 

Procedure. — To  the  solution  of  mixed  chlorides,  add 
5  to  10  c.c.  of  20  per  cent,  perchloric  acid.  (The  amount  of 
perchloric  acid  added  must  be  sufficient  to  convert  all  the 
salts  into  perchlorates.)  Evaporate  the  solution,  preferably  in 
a  glass  basin,  almost  to  dryness.  The  evaporation  is  per- 
formed most  conveniently  on  a  gently  heated  sand-bath, 
and  should  be  continued  until  there  is.  vigorous  evolution 
of  heavy  white  fumes.  If  no  fumes  of  perchloric  acid  appear 
before  the  residue  is  dry,  a  further  quantity  must  be  added, 
and  the  mixture  evaporated  again.  Apart  from  the  risk  of 
loss  by  spirting,  there  is  no  objection  to  evaporating  to 
dryness. 

To  the  almost  dry  residue,  add  about  20  c.c.  of  rectified 
spirit  (about  93  per  cent  alcohol),  and  break  up  the  solid 
thoroughly  with  a  glass  rod.  The  potassium  perchlorate 
alone  remains  undissolved.  Filter  through  a  Gooch  crucible. 
The  asbestos  layer  must  be  much  thicker  than  usual,  as  the 
precipitate  is  sometimes  very  finely  divided.  Wash  with  a 
solution  made  by  adding  2  c.c.  of  20  per  cent,  perchloric 
acid  to  200  c.c.  of  rectified  spirit.  A  separate  small  wash- 
bottle  should  be  used  for  this  solution.  In  most  cases,  about 
150  c.c.  of  the  washing  solution  is  necessary,  but  less  will 
suffice  if  it  is  known  that  the  quantity  of  sodium  or  other 


204          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

soluble  salts  present  is  small.  Finally,  wash  twice  with 
absolute  alcohol,  using  about  5  c.c.  for  each  washing.  Dry 
at  100°  to  120°,  and  weigh. 

Determination  of  Potassium  and  Sodium. 

OUTLINE  OF  METHOD. — All  metals  other  than  sodium  and  potassium 
are  removed.  The  solution  is  evaporated  to  dryness,  and  the  residue 
of  mixed  chlorides  weighed.  The  potassium  is  then  determined  by 
the  perchlorate  method  and  the  sodium  found  by  difference. 

Sodium  Chloride  is  readily  soluble  in  water  but  is  almost 
insoluble  in  alcohol.  It  may  be  dried  completely  at  100°,  but 
unless  the  drying  process  is  very  prolonged,  it  mechanically 
retains  a  trace  of  water  which  is  expelled,  with  decrepitation, 
at  higher  temperatures.  Heated  to  dull  redness  it  melts, 
and  at  a  bright  red  heat  volatilises  rapidly ;  at  all  tempera- 
tures above  the  melting-point,  there  is  appreciable  loss  by 
volatilisation. 

Potassium  Chloride  very  closely  resembles  sodium  chloride 
in  properties. 

Potassium  Perchlorate  has  already  been  described. 

Sodium  Perchlorate  is  a  white  salt  which  is  readily  soluble 
in  water  and  alcohol.  It  may  be  heated  to  about  500° 
without  decomposition ;  at  about  600°  it  melts  and 
decomposes  into  sodium  chloride  and  oxygen.  It  is  not 
decomposed  by  boiling  with  concentrated  hydrochloric 
acid,  but  interacts  with  hot,  concentrated  sulphuric  acid 
with  formation  of  perchloric  acid. 

Procedure. — Evaporate  the  solution  (from  which  sulphate, 
phosphate,  and  all  metals  other  than  the  alkalis  have  been 
removed)  in  a  porcelain  basin  until  the  bulk  is  reduced  to 
about  50  c.c.  Transfer  it  to  a  100  c.c.  platinum  basin  and 
rinse  the  porcelain  basin  with  hot  water.  Evaporate  to 
complete  dryness  on  the  steam-bath.  The  subsequent 
manipulation  is  facilitated  if  the  residue  is  dried  at  this 
stage  as  completely  as  possible,  but  it  is  not  advisable  to 
attempt  to  hasten  the  drying  by  stirring  or  by  breaking  up 
the  mass. 

Place  the  basin  on  a  sand-bath  and  heat — very  gently  at 
first — until  all  moisture  is  driven  off.  During  this  operation, 
the  basin  must  be  kept  covered  with  a  clock-glass  and  the 


POTASSIUM  AND  SODIUM  205 

heating  interrupted  whenever  decrepitation  begins.  When 
decrepitation  has  wholly  ceased,  raise  the  temperature,  but 
do  not  remove  the  clock-glass.  Continue  the  heating  until 
the  clock-glass  and  the  sides  of  the  basin  are  thickly  coated 
with  ammonium  chloride. 

Remove  the  clock-glass.  Invert  it  and  heat  it  gently  over 
a  small  flame  until  all  the  ammonium  chloride  has  volatilised, 
and  set  it  aside  until  required  later. 

Place  the  basin  on  a  pipe-clay  triangle,  and  heat  the  sides 
of  the  basin  until  the  ammonium  chloride  has  volatilised. 
The  burner  must  be  held  in  the  hand  and  the  flame  kept  in 
constant  motion  to  prevent  over-heating  and  consequent 
volatilisation  of  any  alkali  chloride.  Next  heat  the  bottom 
of  the  basin  in  the  same  manner  until  no  more  ammonium 
chloride  is  given  off.  During  this  process  the  residue  almost 
invariably  blackens  owing  to  the  charring  of  traces  of  organic 
impurities  from  the  reagents. 

Cool,  add  about  5  c.c.  of  hot  water,  and  filter  through  a 
very  small  (5  J  cm.)  filter  paper  into  a  tared  platinum  crucible. 
Extreme  care  is  necessary  at  this  stage  as  the  loss  of  a  single 
drop  of  the  solution  renders  the  determination  valueless.  Wash 
the  basin  and  filter  paper  with  hot  water,  using  about  2  c.c. 
for  each  washing.  Wash  into  the  crucible  also  any  trace  of 
salt  adhering  to  the  clock-glass. 

Add  one  drop  of  hydrochloric  acid  and  evaporate  to 
complete  dryness  on  the  steam-bath.  When  the  residue  is 
apparently  dry,  remove  from  the  steam-bath  and  heat  the 
covered  crucible  with  a  small  flame,  observing  the  same 
precautions  against  over-heating  as  before.  Cool,  and  weigh. 
Repeat  the  heating  until  the  weight  is  constant.  (The  salt 
is  sometimes  dark  in  colour  on  account  of  traces  of  carbon. 
The  carbon  will  disappear  on  prolonged  heating,  but  its 
weight  is  negligible.)  The  weight  gives  the  amount  of 
potassium  and  sodium  chlorides. 

Determine  the  amount  of  potassium  in  the  mixed  chlorides 
as  follows  : — To  the  mixed  salts  in  the  crucible,  add  5  c.c.  of 
20  per  cent,  perchloric  acid  for  each  02  gram  of  the  mixture. 
Evaporate  on  a  sand-bath  until  dense  white  fumes  of  per- 
chloric acid  are  evolved,  and  proceed  from  this  stage 
according  to  the  instructions  given  on  p.  203. 


206          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Calculate  the  weight  of  potassium  chloride  corresponding 
to  the  weight  of  potassium  perchlorate  obtained.  Subtract 
the  weight  of  potassium  chloride  from  that  of  the  mixed 
chlorides  in  order  to  find  the  weight  of  sodium  chloride. 

SILICA   AND   SILICATES. 

Properties  of  Silica. — From  the  analytical  point  of  view, 
one  may  distinguish  between  three  varieties  of  silica :  (i)  the 
jelly  obtained  by  the  incomplete  dehydration  of  precipitated 
"  silicic  acid " ;  (2)  silica  obtained  by  the  ignition  of  pre- 
cipitated "  silicic  acid  ";  (3)  native  silica. 

Gelatinous  "  silica  "  is  readily  soluble  in  alkali  hydroxides 
and  carbonates,  and  appreciably  soluble  in  water  and  in 
acids.  After  ignition,  it  is  practically  insoluble  in  water 
and  in  acids  (except  hydrofluoric  acid),  but  dissolves  slowly 
in  alkalis.  Native  crystalline  silica  (e.g.,  quartz)  is 
insoluble  in  acids  (except  hydrofluoric  acid),  and  is  only 
slowly  attacked  by  alkalis. 

The  powder  obtained  by  drying  gelatinous  silica  at  100° 
contains  about  13  percent,  of  water.  Even  at  200°,  it  still 
retains  about  5  per  cent. ;  only  on  ignition  is  the  last  trace 
of  water  expelled.  The  silica  obtained  by  drying  the  jelly 
at  100°  dissolves  to  an  appreciable  extent  in  acid,  and  is  not 
rendered  completely  insoluble  (as  is  often  stated)  by  repeated 
evaporation  to  dryness  with  hydrochloric  acid. 

Precipitated  silica  is  hygroscopic  unless  it  has  been 
ignited  for  at  least  twenty  minutes  with  a  blowpipe  or  a 
Meker  burner. 

Determination  of  Silica  in  an  Insoluble  Silicate. 

OUTLINE  OF  METHOD. — The  silicate  is  decomposed  by  fusion  with 
sodium  carbonate.  The  fused  mass,  after  cooling,  is  disintegrated 
by  warming  with  dilute  hydrochloric  acid,  and  the  solution  is 
evaporated  to  dryness  on  the  steam-bath.  The  residue  is  moistened 
with  concentrated  hydrochloric  acid,  water  is  added,  and  the  silica, 
most  of  which  remains  insoluble,  is  separated  by  filtration.  The 
solution,  which  still  contains  from  I  to  3  per  cent,  of  the  total  silica, 
is  evaporated  to  dryness  again,  and  the  residue  is  treated  as  before. 
The  silica  is  then  dehydrated  by  ignition  and  is  weighed  as  SiO2. 

Procedure. — Weigh  accurately  in  a  platinum  crucible 
about  0-4  gram  of  quartz  (or  about  I  gram  of  a  silicate),  and 


SILICA  AND  SILICATES  207 

mix  it  in  the  crucible  with  about  six  times  as  much  pure, 
dry  sodium  carbonate. 

Heat  the  covered  crucible  with  a  Bunsen  flame.  Keep 
the  flame  moderately  low  at  first,  and  then  very  gradually 
increase  it  to  the  maximum.  Take  care  that  the  action 
does  not  become  violent  through  too  rapid  heating,  and 
cautiously  lift  the  lid  of  the  crucible  at  intervals  in  order  to 
avoid  loss  by  frothing  over.  As  soon  as  the  contents  of  the 
crucible  become  quiescent,  heat  the  crucible  with  a  Meker 
burner  (or  blowpipe)  until  the  molten  mixture  is  almost  clear 
and  little  or  no  effervescence  occurs. 

When  the  fusion  is  complete,  lift  the  crucible  with 
platinum-tipped  tongs  and  impart  a  rotatory  motion  to  the 
vessel  as  cooling  proceeds.  In  this  way,  the  mixture  is 
made  to  solidify  on  the  side  of  the  crucible  and  thus  pre- 
sents a  large  surface ;  this  facilitates  its  subsequent  removal 
from  the  crucible,  but  obviously  care  must  be  taken  not  to 
lose  anything  in  the  process. 

Place  the  crucible,  after  cooling,  in  a  large  porcelain  basin, 
add  water,  and  warm  on  the  steam-bath  until  the  mass  is 
completely  detached  from  the  crucible.  Remove  the  crucible 
and  rinse  it  carefully  with  hot  water.  Gradually  add  excess 
of  moderately  concentrated  hydrochloric  acid,  the  basin 
being  kept  covered  to  prevent  loss  during  the  decomposition 
of  the  carbonate.  Break  down  any  large  lumps  by  gentle 
pressure  with  a  glass  rod,  and,  when  the  disintegration  is 
complete  and  effervescence  ceases,  remove  the  clock-glass 
and  evaporate  to  dryness  on  the  steam-bath.  Towards  the 
end  of  the  evaporation,  stir  constantly  with  a  glass  rod  in 
order  to  break  the  crust  that  forms  on  the  surface  of  the 
liquid.  Evaporation  to  complete  dryness  is  necessary,  and 
care  must  be  taken  not  to  lose  any  of  the  light  powder, 
which  is  very  easily  blown  away. 

Moisten  the  dry  powder  with  10  c.c.  of  concentrated 
hydrochloric  acid,  stir,  and  allow  to  remain  for  ten  minutes 
in  order  that  any  basic  salts  (of  iron,  etc.)  may  be  converted 
into  normal  chlorides.  Then  add  about  30  c.c.  of  water  and 
heat  on  the  steam-bath  and  stir  frequently  until  only  the  silica 
remains  undissolved.  The  silica  is  often  in  a  coarse  condition, 
and  may  be  ground  finer  with  a  pestle  or  a  blunt  glass  rod. 


208          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Filter,  and  wash  several  times  with  cold  water  or  with  hot 
dilute  acid.  (Hot  water  must  not  be  used  since  it  may  cause 
formation  of  insoluble  basic  salts.)  Any  silica  which  adheres 
firmly  to  the  basin  need  not  be  removed  at  this  stage  as  it 
will  be  recovered  later.  Cover  the  filter  with  a  watch-glass, 
and  set  it  aside  until  the  residual  silica  is  recovered  from 
the  filtrate. 

Some  of  the  silica  always  dissolves  in  the  hydrochloric 
acid,  and  the  filtrate  must  therefore  be  evaporated  again 
(using  the  same  basin  since  traces  of  silica  have  been  left 
adhering  to  it)  to  complete  dryness.  Treat  the  residue  with 
hydrochloric  acid  as  before,  dilute,  and  filter  through  the 
paper  containing  the  main  precipitate.  Wash  with  cold  water 
until  no  trace  of  chloride  can  be  detected  in  the  washings. 

It  is  preferable  to  ignite  the  precipitate  without  previous 
drying  as  there  is  less  chance  of  losing  fine  particles  of  silica. 
Fold  the  top  of  the  paper  over  the  precipitate  and  place  in  a 
platinum  crucible,  pressing  down  gently,  but  without  tearing 
the  paper.  Remove  any  trace  of  silica  on  the  surface  of  the 
stirring-rod  or  basin  by  rubbing  with  a  piece  of  moist  filter 
paper,  and  add  this  to  the  main  precipitate.  Heat  gently 
until  dry,  increasing  the  temperature  when  the  paper  begins 
to  char.  Do  not  heat  so  strongly  that  the  escaping  vapour 
catches  fire,  as  the  carbon  burns  more  easily  if  carbonisation 
occurs  at  a  comparatively  low  temperature.  If  there  is  any 
difficulty  in  burning  the  last  of  the  carbon,  remove  the  flame 
from  the  crucible  for  a  minute  or  two,  and  then  re-heat. 
Finally,  ignite  with  a  Meker  burner  for  at  least  twenty 
minutes.  Cool,  and  weigh  the  SiO2. 

The  silica  obtained  in  this  way  is  never  entirely  free 
from  impurities,  and,  in  accurate  work,  it  is  necessary  to 
determine  the  amount  of  impurity  by  driving  off  the  silica 
with  hydrofluoric  acid,  and  weighing  the  non-volatile  residue. 
To  accomplish  this,  moisten  the  silica  with  water,  add  one 
or  two  drops  of  concentrated  sulphuric  acid,  and  3  to 
4  c.c.  of  hydrofluoric  acid  (or  a  little  ammonium  fluoride). 
Evaporate  to  dryness,  first  on  the  steam-bath  and  then  with 
a  Bunsen  flame,  ignite  for  two  or  three  minutes,  cool,  and 
weigh.  (Caution. — Carry  out  the  evaporation  in  a  good 
draught^  and  do  not  inhale  any  hydrofluoric  acid.) 


SILICA  AND  SILICATES  209 

Pure  silica  should  leave  no  residue,  and  the  weight  of  the 
impurity  must  therefore  be  subtracted  from  the  weight  of 
the  crude  silica.  The  residue  is  often  titanium  oxide,  but 
will  also  contain  oxides  of  aluminium,  iron,  and  phosphorus, 
if  these  elements  are  present  in  the  silicate.  If  it  weighs 
more  than  a  milligram,  it  should  be  tested  qualitatively,  or, 
if  a  complete  analysis  of  the  silicate  is  being  made,  the 
subsequent  precipitate  of  alumina,  etc.,  should  be  ignited 
in  the  crucible  containing  the  impurity  found  in  the  silica. 

Notes. — (i)  The  evaporations  for  the  removal  of  silica 
must  be  continued  until  the  silica  forms  a  dry  powder.  This 
powder  is  often  very  light,  and,  like  ignited  silica,  is  very 
easily  blown  away  if  care  is  not  taken  to  protect  it  from 
draughts. 

(2)  It  is  sometimes  stated  that  drying  silica  at  105°  to  1 10° 
renders  it  completely  insoluble.     This  is  incorrect.     If  the 
silica  is  dried  at  a  temperature  much  above  that  of  the  steam- 
bath,  an  increased  amount  of  silica  is  often  carried  into  the 
filtrate  (especially  in  presence  of  much  magnesium),  and  the 
amount  of  impurity  in  the  silica  is  likewise  greater. 

(3)  In  accurate  work,  the  weight  of  the  crude  silica  should 
always   be   corrected   for   impurity.     If,  however,  it   is  not 
intended  to  determine  the  silica  which,  in  spite  of  all  the 
precautions   described   above,  escapes  precipitation,  and   is 
found  along  with  the  ferric  oxide  and  alumina,  the  correction 
should  not  be  applied  ;  instead,  the  assumption  is  made  that 
the  weight  of  the  impurity  is  equal  to  that  of  the  silica  which 
has  passed  into  the  filtrate. 

(4)  The  hydrofluoric  acid  supplied  in  white  paraffin-wax 
bottles  is  usually  pure,  but  it  is  advisable  to  test  a  sample. 
Ammonium  fluoride  may  also  contain  a  non-volatile  impurity. 
If  necessary,  a  correction  must  be  applied  for  the  weight 
of  residue  left  by  the  hydrofluoric  acid  or  the  ammonium 
fluoride  used. 

"Silica"  as  an  Insoluble  Residue. 

In  many  minerals,  slags,  and  technical  products,  the 
residue  left  after  treatment  with  acid  is  mainly  or  altogether 
silica.  Often,  the  portion  which  is  insoluble  in  acid  represents 

O 


210          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

the  silica  which   is  an  accidental  impurity  in  a  mineral  or 
mixture. 

For  technical  purposes,  the  amount  of  substance  undis- 
solved  by  acid  provides  a  sufficiently  accurate  estimate  of 
the  "silica."  Native  silica  is  almost  insoluble  in  dilute 
mineral  acids,  but,  if  very  finely  ground,  considerable 
quantities  may  dissolve  on  prolonged  treatment  with  hot 
acid  solutions.  If,  therefore,  the  other  constituents  of  the 
substance  are  soluble  in  acetic  acid  (of  any  concentration), 
it  is  advisable  to  use  this  acid  in  preference  to  a  mineral 
acid.  If  a  mineral  acid  is  necessary,  use  the  most  dilute 
acid  in  which  the  other  constituents  can  be  dissolved. 

Typical  examples  of  the  determination  of  "  silica,"  when 
it  is  the  only  portion  insoluble  in  acid,  are  described  under 
Dolomite  (p.  228)  and  Pyrites  (p.  239).  In  these  determina- 
tions some  of  the  silica  usually  dissolves ;  but,  on  the  other 
hand,  the  residue  is  not  entirely  silica,  and  for  many  purposes 
the  uncorrected  results  are  sufficiently  accurate. 

When  an  accurate  determination  of  the  silica  is  required, 
the  following  modifications  are  necessary: — (i)  The  "silica" 
must  be  tested  for  impurities  by  evaporation  with  hydro- 
fluoric acid  and  a  drop  of  concentrated  sulphuric  acid 
(p.  208);  if  quartz  is  present,  more  than  one  evaporation 
with  hydrofluoric  acid  may  be  necessary  to  volatilise  it. 
If  the  residue  does  not  exceed  2  or  3  mgrms.,  it  is  assumed  to 
be  alumina,  ferric  oxide,  titanium  oxide,  barium  sulphate,  or 
a  phosphate — the  nature  of  the  original  substance  being  the 
best  guide  as  to  which  of  these  impurities  are  the  most 
probable.  If  the  impurity  in  the  silica  exceeds  about  3  mgrms., 
it  should  be  fused  with  sodium  carbonate  and  added  to  the 
main  solution.  (2)  The  solution  may  contain  an  appreci- 
able amount  of  silica;  it  must  therefore  be  evaporated  to 
complete  dryness  and  the  silica  separated  as  previously 
described  (p.  207).  One  evaporation  will  be  sufficient  to 
remove  practically  all  the  silica  in  this  case. 

Determination  of  Silica  in  a  Soluble  Silicate. 

Sodium  silicate  is  the  only  common  soluble  silicate.  A 
solution  of  crude  sodium  silicate  is  sold  as  "  water-glass."  It 
is  decomposed  by  acid  with  precipitation  of  the  silica. 


SILVER— SODIUM— SULPHATE  211 

Method. — Hydrochloric  acid  is  added  in  excess  and  the 
solution  is  evaporated  to  complete  dryness.  The  residue  is 
extracted  with  hydrochloric  acid  and  water,  and  the  solution, 
after  filtration,  is  again  evaporated  to  dryness  in  order  to 
render  the  remainder  of  the  silica  insoluble.  The  details  of 
the  process  are  given  above,  the  treatment  being  exactly  the 
same  as  that  of  the  solution  obtained  after  the  fusion  of  an 
insoluble  silicate. 

The  second  evaporation  to  dryness  must  not  be  orqitted. 

SILVER. 

The  volumetric  method  for  the  determination  of  silver, 
described  on  p.  104,  is  convenient  and  accurate. 

Gravimetrically,  silver  is  usually  determined  as  chloride, 
but  the  gravimetric  determination  as  silver  bromide  may  be 
recommended. 

Forms  in  which  Silver  is  Precipitated. 

Silver  Chloride. — By  this  method  silver  may  be 
determined  in  presence  of  all  other  metals.  If  a  mercurous 
salt  is  present,  it  must  be  oxidised  with  concentrated  nitric 
acid  prior  to  the  precipitation  of  the  silver.  If  lead  is 
present,  the  solution  must  be  diluted  and  the  hydrochloric 
acid  added  very  slowly. 

The  silver  is  precipitated  with  dilute  hydrochloric  acid, 
carefully  avoiding  unnecessary  excess.  Otherwise  the 
procedure  is  identical  with  that  adopted  in  determining 
chloride  as  silver  chloride  (p.  133). 

Silver  Bromide. — This  method  is  preferable  to  the 
chloride  method  on  account  of  the  lower  solubility  of  silver 
bromide.  The  solution  is  acidified  with  nitric  acid  and 
potassium  bromide  added  until  precipitation  is  complete. 
The  procedure  is  otherwise  identical  with  the  previous 
method.  For  the  properties  of  silver  bromide,  see  p.  173. 

SODIUM  (see  p.   199) 
SULPHATE. 

There  is  no  convenient  volumetric  method  for  the 
determination  of  sulphate. 


212          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Sulphate  is  always  determined  gravimetrically  as  barium 
sulphate.  For  details,  see  p.  131. 

SULPHIDE. 

A  volumetric  method  for  the  determination  of  hydrogen 
sulphide  is  described  on  p.  91.  Many  sulphides  are  readily 
decomposed  by  dilute  acids,  and  the  volumetric  method 
may  therefore  be  adapted  to  their  determination. 

In  order  to  determine  sulphide  gravimetrically,  a  weighed 
sample  (or  measured  volume)  is  decomposed  with  hydro- 
chloric acid  in  an  apparatus  similar  to  that  shown  in  Fig. 
2%>  P-  93-  To  prevent  liberation  of  sulphur  by  atmospheric 
oxidation,  the  apparatus  must  be  filled  with  carbon  dioxide 
or  hydrogen.  The  hydrogen  sulphide  is  led  into  a  solution 
of  ammonia  and  hydrogen  peroxide,  and  is  thereby  oxidised 
to  sulphate.  The  sulphate  is  determined  in  the  usual 
manner  as  barium  sulphate. 

The  oxidation  of  the  hydrogen  sulphide  may  also  be 
effected  by  absorbing  the  gas  in  sodium  hydroxide 
solution,  and  then  adding  bromine. 

Other  methods  of  carrying  out  the  oxidation  are  described 
in  connection  with  the  analysis  of  pyrites  (pp.  240  and  242). 

TIN. 

Tin  is  a  constituent  of  many  alloys;  apart  from  these, 
it  will  rarely  be  met  with  in  analysis  except  in  cassiterite, 
which  is  mainly  stannic  oxide. 

The  tin  in  alloys  may  be  determined  by  one  or  other  of 
the  following  methods  : — 

(1)  The  alloy   is   disintegrated  with  nitric  acid,  the  tin 
remaining   as  insoluble  stannic  oxide.     This  method  gives 
inaccurate  results  unless  the  stannic  oxide  is  further  examined 
for  traces  of  other  oxides  which  are  pertinaciously  retained 
by  it.     If  the  alloy  contains  arsenic,  antimony,  or  phosphorus, 
the  method  requires  considerable  modification.      Details  of 
the   procedure  are  given  on  p.  223  in  connection  with  the 
analysis  of  solder. 

(2)  The  alloy  is  dissolved  in  a  mixture  of  concentrated 
sulphuric  and  nitric  acids.     The  solution  is  then  diluted  and 


TIN— WATER  213 

boiled,  whereby  all  the  tin  is  precipitated  as  pure  stannic 
oxide.  Details  of  the  procedure  are  given  on  p.  224  in 
connection  with  the  analysis  of  a  bronze.  This  method 
should  not  be  used  for  alloys  containing  a  high  percentage 
of  lead. 

(3)  The  alloy  is  dissolved  in  hydrochloric  acid,  and  the 
tin  is  determined  volumetrically,  as  described  on  p.  95. 

WATER. 

Three  methods,  with  many  modifications  of  each,  are 
used  for  the  determination  of  water.  Each  method  has 
advantages  for  particular  cases,  and  the  accuracy  of  the 
determination  often  depends  on  the  choice  of  the  appropriate 
method. 

(1)  Indirect  Method. — A  weighed  sample  is  heated  to  a 
high  temperature  and  the  loss  of  weight  determined.     This 
is  the  easiest  method,  but  is  only  accurate  (a)  if  nothing 
but  water  is  lost  on  heating,  and  (b)  if  no  chemical  change, 
such  as  the  oxidation  of  a  ferrous  salt,  occurs  during  the 
process. 

(2)  Direct  Method. — A   weighed  sample  is   heated   and 
the   water   evolved   is   collected    and    weighed.     There   are 
many  modifications  of  the  methods  of  heating  the  substance 
and  of  collecting   the   water.     The  direct   method   is  more 
generally    applicable    than    the     indirect    method,    but     is 
somewhat  more  troublesome. 

(3)  Carbide  Method. — A  weighed  sample   is   intimately 
mixed  with  calcium  carbide  and  the  liberated  acetylene  is 
measured.     The  application  of  heat   may   or   may   not   be 
necessary. 

A  modification  of  this  method  consists  in  determining 
the  loss  of  weight  when  weighed  quantities  of  the  substance 
and  of  calcium  carbide  are  mixed.  For  details,  see  papers 
by  F.  H.  Campbell  (J.  Soc.  Chem.  Industry,  1913,  32,  67, 
where  a  full  list  of  references  to  other  modifications  is  given), 
and  Huntly  and  Coste  (J.  Soc.  Chem.  Industry \  1913,  32,  6 1). 


214  SYSTEMATIC  QUANTITATIVE  ANALYSIS 


Indirect  Determination  of  Water. 

As  this  is  the  easiest  method,  it  is  used  whenever  possible. 
It  is  inaccurate  : — 

(1)  If   anything    except   water    is    lost   during    the    de- 

hydration. This  is  particularly  liable  to  occur  with 
carbonates,  organic  substances,  and  ammonium  com- 
pounds. 

(2)  If  the  substance   is  readily  oxidised.     The  method 
therefore   gives   inaccurate   results,    for   example,   if 
ferrous  salts  are  present. 

The  first  error  can  be  avoided  in  some  cases  by  dehydrating 
at  a  low  temperature  by  means  of  a  current  of  dry  air  or  in 
a  vacuum.  The  second  error  is  avoided  by  dehydrating  in 
an  oxygen-free  atmosphere  or  in  a  vacuum. 

Three  of  the  many  modifications  of  the  indirect  method 
may  be  mentioned. 

(1)  Gentle  ignition  until  constant  weight  is  attained.     The 
procedure  has  already  been  described  for  the  determination 
of  water  in  magnesium  sulphate  heptahydrate  (p.  123). 

(2)  Drying  in  a  steam-oven  or  hot  air-oven  at  constant 
temperature.     Most  hydrated  salts  can  be  dried  at  tempera- 
tures between  100°  and  200°  without  further  decomposition. 
There  is  no  fixed  temperature  at  which  hydrated  salts  will 
become  anhydrous,  and  it  is  therefore  necessary  to  find  by 
trial   the  temperature,   if  any,  at  which   the  water   can    be 
expelled  without  further  decomposition  of  the  substance. 

The  substance  is  weighed  in  a  wide,  shallow,  weighing- 
bottle,  and  the  open  bottle  is  placed  in  the  hot  air-oven  for 
one  hour.  The  bottle  is  then  removed  and  cooled  in  a 
desiccator.  The  stopper  must  be  replaced  before  weighing, 
as  reabsorption  of  moisture  may  occur.  The  procedure  is 
repeated  until  the  weight  is  constant. 

(3)  Most  substances  may  be  dried  without  decomposition 
in  a  vacuum   desiccator  which   contains   sulphuric  acid   or 
fused    calcium  chloride.     The  substance  should  be  weighed 
in  a  wide  shallow  bottle  or  on  a  watch-glass.     Dehydration 
may  occur  so  slowly  at   the  ordinary  temperature   that   it 
only  becomes  complete  after  many  days;   if  the  substance 


WATER  215 

is   heated    in  vacuo,   dehydration    occurs  very  quickly   and 
there  is  no  danger  of  oxidation. 

Direct  Determination  of  Water  in  a  Mineral. 

If  no  other  volatile  constituent  is  present,  the  following 
simple  method  (Penfield's  method)  gives  very  accurate  results. 

The  mineral  is  heated  in  a  hard-glass  (or  Jena  glass) 
tube,  which  is  enlarged  into  a  bulb  A  at  the  closed  end. 
This  tube  should  be  about  20  cm.  long  and  about  5  mm.  in 
diameter.  One  or  more  bulbs  should  be  provided  about  the 
middle  of  the  tube  in  order  to  catch  the  water  and  prevent 
it  running  back  and  cracking  the  hot  glass  (Fig.  55). 


D 


B 

FIG.  55. 

Even  if  apparently  dry,  these  tubes  must  be  thoroughly 
dried  before  use,  by  heating  and  blowing  air  through  them 
by  means  of  a  narrow  glass  tube  reaching  to  the  bottom. 

Procedure. — Introduce,  by  means  of  the  tube  B,  a  weighed 
quantity  of  the  mineral,  and  heat  with  a  Bunsen,  or,  if  neces- 
sary, with  a  Meker  burner,  the  tube  being  clamped  in  a 
horizontal  position.  To  prevent  loss  of  steam  by  air  cur- 
rents, close  the  open  end  of  the  tube  by  a  stopper  C,  made 
from  a  piece  of  tubing  drawn  out  to  a  capillary  and  fitted 
with  rubber  tubing.  The  water  condenses  in  the  middle 
bulbs,  which  are  kept  cool  by  strips  of  moistened  filter  paper. 

If  prolonged  heating  is  necessary,  a  screen  of  asbestos 
board  between  the  middle  bulbs  and  the  flame  is  desirable. 

Finally  draw  off  the  heated  end  at  about  D,  and  weigh 
the  tube  after  cooling  and  external  cleansing.  Remove  the 
water  by  aspiration  and  weigh  the  tube  again. 

By  the  addition  of  a  suitable  "  retainer,"  such  as  calcium 
oxide,  the  method  can  be  used  even  when  the  substance 
contains  fluorine,  sulphur,  etc.  (see  Amer.  Jour.  Sci.}  3rd 
series,  48,  31,  1894). 


216          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

Exercise. — Determine  the  percentage  of  water  in  gypsum 
or  in  barium  chloride,  heating  with  the  Bunsen  flame  only. 

ZINC. 

The  volumetric  method  for  the  determination  of  zinc 
(p.  108)  is  more  expeditious  but  less  accurate  than  the 
gravimetric  methods. 

Zinc  is  always  determined  after  removal  of  metals  pre- 
cipitated by  hydrogen  sulphide  in  acid  solution.  Attention 
may  be  directed  to  the  fact  that  zinc  is  partly  precipitated, 
together  with  metals  of  the  copper  group,  unless  the  solution 
is  very  strongly  acid,  and,  even  then,  reprecipitation  may  be 
necessary  to  remove  the  last  of  the  zinc. 

Forms  in  -which  Zinc  is  precipitated. 

Basic  Zinc  Carbonate. — This  method  is  applicable  only 
when  all  metals  other  than  sodium  and  potassium  are  absent. 
It  is  inaccurate  in  presence  of  ammonium  salts,  but  these  can 
be  removed  before  precipitation.  For  details,  see  p.  137. 

Zinc  Ammonium  Phosphate. — This  method  is  available 
in  presence  of  sodium,  potassium,  and  ammonium  salts  only. 

Zinc  Sulphide. — Zinc  may  be  separated  from  the  calcium 
group  by  precipitation  as  sulphide,  and  in  general  analysis 
this  is  frequently  the  only  available  method.  As,  however, 
it  is  a  matter  of  considerable  difficulty  to  obtain  zinc  sulphide 
in  a  form  suitable  for  filtration,  it  is  preferable,  when 
circumstances  permit,  to  precipitate  as  basic  carbonate  or 
as  phosphate. 

A  complete  separation  of  zinc  from  iron,  aluminium, 
manganese,  and  nickel  is  obtained  by  precipitation  as 
sulphide  in  presence  of  formic  acid. 

Determination  of  Zinc  as  Phosphate. 

OUTLINE  OF  METHOD. — The  zinc  is  precipitated  as  zinc  ammonium 
phosphate  by  means  of  ammonium  phosphate  in  neutral  or  very 
faintly  acid  solution.  It  is  weighed  either  as  Zn(NH4)PO4  after 
drying  at  120°,  or  as  Zn2P2O7  after  ignition. 

Zinc  Ammonium  Phosphate  is  a  white  crystalline  powder, 
insoluble  in  water  and  in  solutions  of  ammonium  salts,  but 


ZINC  217 

somewhat  soluble  in  ammonia.  It  is  readily  soluble  in 
mineral  acids,  but  is  insoluble  in  very  dilute  acetic  acid. 
It  may  be  dried  without  decomposition  at  temperatures  not 
exceeding  140° ;  heated  above  200°,  it  is  converted  into  zinc 
pyrophosphate,  Zn2P2O7. 

Zinc  Pyrophosphate  is  a  white  powder  which  may  be  heated 
to  dull  redness  without  decomposition.  Flame  gases  and 
carbonaceous  matter  must  be  carefully  excluded  during  the 
ignition,  otherwise  reduction  and  volatilisation  will  occur. 

Procedure  (in  absence  of  sodium  and  potassium). — 
Evaporate  the  solution  to  100  c.c.,  cool,  add  5  grams  of 
ammonium  phosphate  (the  di-ammonium  salt)  dissolved  in 
water,  and  then  add  ammonia  until  the  solution  is  neutral 
(test  with  litmus  paper).  Add  i  c.c.  of  dilute  acetic  acid, 
and  stir.  Heat  on  the  steam-bath  for  an  hour ;  in  that  time 
the  precipitate  should  be  crystalline  and  should  have  settled 
completely.  Filter  through  a  Gooch  crucible,  wash  with  hot 
water,  dry  at  110°  to  120°,  and  weigh  as  Zn(NH4)PO4. 

The  above  method  is  recommended  as  accurate,  but  if  it 
is  desired  to  weigh  as  pyrophosphate,  the  Gooch  crucible 
must  be  placed  inside  a  platinum  or  nickel  crucible  and 
ignited — at  first  gently,  but  finally  to  redness.  Care  must 
be  taken  to  exclude  flame  gases  during  the  ignition,  as 
reduction  (with  volatilisation  of  the  zinc)  occurs  readily. 

Note. — The  above-mentioned  conditions  of  acidity  during 
the  precipitation  must  be  rigidly  adhered  to,  otherwise 
precipitation  is  incomplete. 

Modification  if  Alkalis  are  present. — If  sodium  or 
potassium  salts  are  present,  even  in  small  amount,  the 
precipitate  obtained  is  a  mixture  of  zinc  ammonium 
phosphate  and  zinc  potassium  (or  sodium)  phosphate. 

When  these  salts  are  present,  add  20  grams  of  ammonium 
chloride  to  the  solution  and  precipitate,  as  described  above. 
When  the  precipitate  has  settled,  decant  through  a  Gooch 
crucible  and  wash  three  times  with  hot  water  by  decantation, 
care  being  taken  that  as  little  as  possible  of  the  precipitate 
is  washed  into  the  crucible. 

Dissolve  the  precipitate  in  the  beaker  in  the  minimum 
amount  of  dilute  hydrochloric  acid,  add  10  grams  of 


218          SYSTEMATIC  QUANTITATIVE  ANALYSIS 

ammonium  chloride,  and  repeat  the  process  of  neutralisa- 
tion and  precipitation.  Filter  through  the  Gooch  crucible 
used  in  the  first  operation,  wash  thoroughly,  ignite,  and 
weigh  as  Zn2P2O7. 

It  is  necessary  in  this  case  to  ignite  to  pyrophosphate,  as, 
in  presence  of  large  amounts  of  ammonium  salts,  the  zinc 
ammonium  phosphate  is  contaminated  with  ammonium  salts 
which  are  not  completely  removed  by  washing. 

If  only  sodium  salts  are  present,  a  single  precipitation  in 
presence  of  a  large  amount  of  ammonium  chloride  is  sufficient. 
The  precipitate  must  be  converted  into  pyrophosphate. 

Determination  of  Zinc  as  Sulphide. 

(Precipitation  in  presence  of  Formic  Acid.} 

OUTLINE  OF  METHOD. —  The  zinc  is  precipitated  as  sulphide  by 
hydrogen  sulphide  in  presence  of  a  small  amount  of  formic  acid. 
The  sulphide  is  converted  into  oxide  by  ignition  in  air  and  the  ZnO 
weighed,  or  is  ignited  in  hydrogen  and  weighed  as  ZnS. 

Zinc  Sulphide,  obtained  by  precipitation,  is  a  hydrated 
gelatinous  substance.  It  is  readily  soluble  in  strong  acids, 
insoluble  in  ammonia  and  in  alkaline  solutions  generally,  and 
almost  insoluble  in  dilute  solutions  of  acetic  or  formic  acids. 
Whatever  the  conditions  of  precipitation,  it  is  somewhat 
difficult  to  filter.  The  best  precipitate  is  obtained  from  an 
acid  solution  ;  it  then  filters  fairly  readily,  but  if  washed  with 
water,  becomes  more  gelatinous  and  chokes  the  filter  paper. 
It  can,  however,  be  washed  with  dilute  solutions  of  ammonium 
salts. 

Zinc  sulphide  is  quickly  oxidised  to  zinc  oxide  on  ignition 
in  air,  and  is  therefore  usually  converted  into  oxide  for 
weighing ;  it  can,  however,  be  dried  and  obtained  as 
anhydrous  ZnS  by  gentle  ignition  with  sulphur  in  an 
atmosphere  of  hydrogen,  using  a  Rose  crucible. 

Zinc  Oxide. — The  properties  of  zinc  oxide  are  described 
on  p.  137. 

Procedure. — To  the  zinc  solution  contained  in  a  400  c.c. 
conical  flask,  add  i  c.c.  of  methyl  orange,  and  then  run  in 
carefully  a  dilute  solution  of  sodium  hydroxide  until  the  last 


ZINC  219 

tinge  of  pink  is  discharged  (avoid  excess).  Dilute  5  c.c.  of 
ordinary  95  per  cent,  formic  acid  to  100  c.c.,  and  add  this  to 
the  zinc  solution  until  a  faint  permanent  pink  colour  is 
obtained,  and  then  add  an  additional  5  c.c.  of  the  5  per 
cent,  formic  acid. 

Dilute  the  solution  to  about  250  c.c.,  heat  to  80°,  and 
saturate  with  hydrogen  sulphide  under  slight 
pressure.  Pressure  a  little  above  atmospheric 
is  readily  obtained  with  an  ordinary  Kipp 
apparatus  if  the  flask  is  fitted  with  a  rubber 
cork  (Fig.  56).  The  cork  is  inserted  firmly 
into  place  after  the  air  in  the  flask  has  been 
displaced  by  hydrogen  sulphide.  When  the 
solution  is  saturated  and  the  precipitate  has  IG*  5  * 

settled,  the  cork  is  removed  before  the  apparatus  is  discon- 
nected elsewhere. 

Decant,  filter  with  slight  suction,  and  wash  with  a 
saturated  hydrogen  sulphide  solution  containing  2  per  cent, 
of  ammonium  acetate  ;  wash  finally  with  hot  water. 

Dry  the  precipitate  and  filter  paper  thoroughly.  Remove 
the  precipitate  as  completely  as  possible  from  the  filter  paper, 
but  do  not  rub  off  any  paper  fluff,  as  this  would  cause 
reduction  and  loss  of  zinc  in  the  subsequent  ignition. 

If  the  sulphide  is  to  be  weighed,  incinerate  the  paper  in 
a  Rose  crucible  before  adding  the  precipitate.  Add  a  little 
pure  sulphur,  ignite  at  a  low  red  heat  in  a  current  of 
hydrogen,  and  weigh  the  ZnS. 

If  the  sulphide  is  to  be  converted  into  oxide,  incinerate  the 
paper  before  adding  the  precipitate,  and  ignite  in  an  open 
crucible  with  careful  exclusion  of  flame  gases.  Weigh 
the  ZnO. 


PART   VI 

THE    ANALYSIS   OF    SIMPLE    ORES 
AND   ALLOYS 

ANALYSIS    OP    A   SILVER   COIN. 

(Alloy  of  Silver  and  Copper?) 

OUTLINE  OF  METHOD.— The  coin  is  dissolved  in  nitric  acid.  The 
silver  is  precipitated  as  silver  chloride,  which  is  separated  by 
filtration,  dried,  and  weighed.  The  copper  is  determined  in  the 
filtrate  as  cupric  oxide  or  as  cuprous  sulphide. 

EUROPEAN  silver  coins  contain,  as  a  rule,  from  90  to  95  per 
cent,  of  silver,  the  remainder  being  copper.  The  amount 
taken  for  analysis  must  be  sufficient  for  an  accurate 
determination  of  the  copper,  and  the  coin  or  portion  of  a 
coin  taken  for  analysis  should  therefore  weigh  i-o  to  1-5 
grams.  For  exercise,  use  a  new  threepenny  piece. 

Procedure. — Clean  the  coin  with  emery  cloth,  weigh  it 
accurately,  and  place  it  in  a  400  c.c.  beaker,  provided  with 
a  cover-glass.  Add  a  mixture  of  10  c.c.  of  concentrated  nitric 
acid  and  5  c.c.  of  water.  When  solution  is  complete,  rinse 
and  remove  the  cover-glass,  and  evaporate  the  solution 
nearly  to  dryness  on  the  steam-bath.  Dilute  to  about 
100  c.c.,  and  determine  the  silver  as  chloride.  For  details 
of  the  procedure,  see  pp.  211  and  133. 

Combine  the  filtrate  and  washings  from  the  silver  chloride 
precipitation,  and  determine  the  copper  either  as  cupric  oxide 
or  as  cuprous  sulphide.  If  the  copper  is  to  be  determined 
as  cupric  oxide,  proceed  as  directed  on  p.  139,  without 
removing  the  nitrate.  If  the  copper  is  to  be  determined 
as  cuprous  sulphide,  proceed  as  follows. 

Add  5  c.c.  of  dilute  sulphuric  acid  to  the  solution,  and 
evaporate  to  dryness  on  the  steam-bath.  Dissolve  the 
residue  in  a  few  drops  of  dilute  sulphuric  acid,  dilute  with 


NICKEL  COIN  221 

water,  and  determine  the  copper  in  the  solution  as  cuprous 
sulphide  (p.  141). 

Alternative  Method. — The  analysis  of  a  silver  coin  may 
also  be  performed  by  volumetric  methods. 

Dissolve  the  alloy  (i-o  to  1-5  grams)  in  nitric  acid  as 
described  above,  dilute  the  solution  with  a  little  water,  and 
boil  for  ten  minutes.  Dilute,  in  a  standard  flask,  to  250  c.c. 
By  means  of  a  dry  pipette,  withdraw  50  c.c.  for  the  deter- 
mination of  the  silver  (p.  104). 

Determine  the  amount  of  copper  in  the  remainder  of 
the  solution  (200  c.c.)  by  means  of  decinormal  sodium  thio- 
sulphate  solution.  Transfer  the  solution  to  a  beaker,  add 
ammonia  until  a  blue  precipitate  separates,  and  boil  for  a 
few  minutes ;  then  add  acetic  acid  until  the  precipitate  has 
redissolved,  and  boil  again  for  a  few  minutes.  Cool,  add 
about  5  grams  of  potassium  iodide  dissolved  in  a  little 
water,  and  titrate  the  iodine  with  decinormal  thiosulphate, 
as  described  on  p.  89.  The  precipitated  silver  iodide  does 
not  interfere  with  the  titration. 

ANALYSIS   OP   A   GERMAN   NICKEL   COIN. 

(Alloy  of  Copper  and  Nickel?) 

German  "  nickel "  coins  are  made  of  an  alloy  containing 
about  three  parts  of  copper  to  one  of  nickel,  together  with 
a  trace  of  iron.  Several  methods  of  analysis  are  available, 
one  of  the  most  accurate  being  the  electrolytic  method 
(see  p.  153).  The  following  method  also  yields  very 
accurate  results. 

OUTLINE  OF  METHOD. — The  alloy  is  dissolved  in  a  mixture  of  sulphuric 
and  nitric  acids.  The  solution  is  evaporated  until  all  the  nitric  acid 
is  expelled,  water  is  added  and  sulphuric  acid  sufficient  to  make  the 
concentration  of  acid  20  per  cent.,  by  volume.  The  copper  is  pre- 
cipitated by  hydrogen  sulphide,  and  is  then  converted  into  cuprous 
sulphide,  which  is  weighed.  By  addition  of  sodium  hydroxide  and 
bromine  to  the  filtrate,  the  nickel  is  precipitated  as  nickelic  hydroxide. 
This  is  converted  into  metallic  nickel,  which  is  weighed.  Any  iron 
in  the  alloy  is  obtained  in  the  form  of  metallic  iron  as  an  impurity 
in  the  nickel ;  it  is  determined  volumetrically,  and  a  correction 
applied  to  the  nickel. 

Although  nickel  is  not  precipitated  by  hydrogen  sulphide 
from  acid  solutions  of  pure  nickel  salts,  some  nickel  is 


222       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

always  co-precipitated  with  sulphides  of  the  copper  group, 
unless  the  solution  is  strongly  acid.  The  more  acid  the 
solution,  the  less  nickel  is  precipitated.  A  practically 
complete  separation  may  be  obtained  by  precipitating  from 
a  solution  containing  20  per  cent,  by  volume,  of  sulphuric 
acid.  Copper  is  completely  precipitated,  even  in  presence 
of  this  amount  of  acid,  if  the  solution  is  saturated  with 
hydrogen  sulphide. 

Procedure. 

Clean  a  5  or  10  pfennig  piece  with  emery  cloth,  and  cut 
it  into  small  pieces  with  shears.  Place  a  weighed  portion 
(0-5  to  07  gram)  in  a  300  c.c.  conical  flask,  and  add  15  c.c. 
of  water,  5  c.c.  of  concentrated  sulphuric  acid,  and  5  c.c.  of 
concentrated  nitric  acid.  Cover  the  flask,  and  warm  gently 
until  all  the  alloy  has  dissolved.  Rinse  the  cover-glass  with 
a  little  water,  and  boil  the  contents  of  the  flask  until  all  the 
nitric  acid  is  expelled  and  copious  white  fumes  of  sulphuric 
acid  are  evolved.  Cool;  add  85  c.c.  of  water  and  15  c.c.  of 
concentrated  sulphuric  acid. 

Determination  of  Copper. — Saturate  the  solution  with 
hydrogen  sulphide  by  passing  a  slow  current  of  the  gas  for 
some  hours.  Filter,  and  wash  with  hydrogen  sulphide 
solution,  observing  the  precautions  against  oxidation 
mentioned  on  p.  142.  Dry  the  precipitate,  convert  it  into 
cuprous  sulphide,  and  weigh. 

Determination  of  Nickel. — Combine  the  filtrate  and 
washings,  and  evaporate  until  the  hydrogen  sulphide  is 
completely  expelled.  Determine  the  nickel  as  described 
on  p.  195.  The  metallic  nickel  obtained  in  this  way  is 
always  contaminated  with  silica  (from  the  vessels  employed) 
and  with  any  trace  of  iron  present  in  the  coin.  Dissolve 
the  crude  nickel  in  the  minimum  amount  of  nitric  acid,  and 
filter  from  the  residue  of  silica.  In  the  filtrate,  determine 
the  amount  of  iron  voiumetrtcally,  as  described  on  p.  76. 
(Before  reducing  the  iron,  the  ferric  nitrate  must  be  converted 
into  sulphate  by  evaporation  with  sulphuric  acid  until  dense 
white  fumes  are  evolved.)  Subtract  the  weights  of  silica 
and  iron  from  the  weight  of  crude  nickel,  in  order  to  obtain 
that  of  the  pure  nickel. 


SOLDER  223 

ANALYSIS   OP   SOLDER. 

(Alloy  of  Tin  and  Lead.) 

OUTLINE  OF  METHOD. — The  alloy  is  disintegrated  by  treatment  with 
concentrated  nitric  acid,  which  converts  the  lead  into  soluble  lead 
nitrate  and  the  tin  into  insoluble  stannic  oxide.  The  stannic  oxide 
is  separated  by  filtration,  and  the  lead  in  solution  is  determined  as 
chromate  or  sulphate.  The  stannic  oxide,  which  always  contains 
some  lead,  is  dried,  ignited,  and  weighed.  The  amount  of  lead  in 
the  impure  stannic  oxide  is  then  determined,  and  a  correction  for 
this  is  applied  to  both  tin  and  lead. 

Procedure. — Weigh  accurately  about  0-3  gram  of  the 
alloy  which  has  been  rolled  into  a  thin  sheet.  Place  the 
weighed  portion  in  a  porcelain  basin,  add  about  5  c.c.  of 
concentrated  nitric  acid  and  cover  the  basin  with  a  clock- 
glass.  Then  add  water,  drop  by  drop,  in  just  sufficient 
amount  to  start  the  reaction.  (The  more  concentrated  the 
acid,  the  less  lead  will  be  retained  by  the  stannic  oxide.) 
Heat  gently  when  the  action  becomes  slow,  but  add  no  more 
water  until  all  the  metal  has  been  disintegrated.  Then  add 
15  to  20  c.c.  of  water,  and  boil  gently  for  five  minutes.  Filter, 
and  wash  the  insoluble  residue  thoroughly  with  hot  water. 

Determine  the  lead  in  the  filtrate  as  chromate  or  as 
sulphate  (p.  187). 

After  drying  the  stannic  oxide,  incinerate  the  filter  paper 
in  a  porcelain  crucible  apart  from  the  precipitate.  When 
all  the  carbon  has  been  burnt  off,  add  the  stannic  oxide, 
moisten  with  a  drop  of  concentrated  nitric  acid  to  reoxidise 
traces  of  reduced  oxide,  and  ignite  strongly  until  of  constant 
weight.  The  stannic  oxide  obtained  in  this  way  always 
contains  some  lead.  If  the  above  procedure  has  been 
carefully  followed,  the  amount  of  lead  will  be  very  small, 
but  it  should,  in  all  cases,  be  determined. 

After  weighing  the  impure  stannic  oxide,  mix  it  with  six 
times  its  weight  of  equal  parts  of  pure  sulphur  and  anhydrous 
sodium  carbonate.  Heat  the  mixture  in  a  covered  crucible 
until  there  is  no  longer  any  odour  of  sulphur  dioxide.  Cool, 
boil  with  a  little  water,  and  filter.  The  tin  dissolves  as 
sodium  thiostannate  and  the  lead  remains  as  insoluble 
sulphide.  Wash  with  dilute  sodium  sulphide  solution  and 
then  with  hydrogen  sulphide  solution.  Not  more  than  a 


221       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

few  milligrams  of  a  fine  black  powder  should  remain  on  the 
filter  paper ;  if  the  residue  is  granular  or  if  it  is  light  in 
colour,  it  must  be  again  fused  with  sulphur  and  sodium 
carbonate.  Dry  and  ignite  the  small  black  residue.  Cool, 
add  one  drop  of  concentrated  sulphuric  acid,  heat  gently 
until  dry,  and  weigh  the  lead  sulphate  obtained.  Calculate 
the  weight  of  lead  oxide  which  is  equivalent  to  the  lead 
sulphate  obtained,  and  subtract  this  from  the  weight  of  the 
impure  stannic  oxide,  in  order  to  obtain  the  weight  of  pure 
stannic  oxide.  Also,  add  to  the  lead  chromate  (or  lead 
sulphate)  an  amount  corresponding  to  the  weight  of  lead 
oxide  found  with  the  stannic  oxide. 

ANALYSIS   OP   A   BRONZE. 

A  bronze  is  an  alloy  consisting  essentially  of  copper  and 
tin,  but  usually  containing  also  some  zinc,  together  with 
traces  of  lead,  nickel,  and  iron.  Gun  metal  is  composed 
nominally  of  ninety  parts  of  copper  and  ten  parts  of  tin. 
English  bronze  coins  contain  about  ninety-five  parts  of  copper, 
four  parts  of  tin,  and  one  part  of  zinc.  The  amount  of 
lead  in  bronze  coins  rarely  exceeds  o-i  per  cent,  but  much 
larger  proportions  of  lead  are  present  in  some  varieties  of 
bronze.  Aluminium  bronze  usually  consists  of  about  ninety 
parts  of  copper  and  ten  parts  of  aluminium,  but  may  contain 
tin  and  other  metals. 

In  order  to  make  the  description  general,  it  is  assumed 
that  the  alloy  under  analysis  contains  copper,  tin,  and  zinc, 
with  traces  of  iron  and  lead.  The  method  described  will 
therefore  apply  to  most  varieties  of  bronze,  slight  modifications 
being  necessary  when  aluminium  or  nickel  is  present. 

OUTLINE  OF  METHOD. — The  alloy  is  dissolved  in  a  mixture  of 
concentrated  sulphuric  and  nitric  acids.  The  solution  is  evaporated 
until  all  the  nitric  acid  is  expelled,  diluted  until  the  sulphuric  acid 
concentration  is  about  7  per  cent,  by  volume,  and  the  precipitated 
lead  sulphate  is  removed  by  filtration.  Copper  is  removed  by 
electrolysis.  The  solution  is  largely  diluted,  and  boiled,  whereby 
the  tin  is  precipitated  as  pure  stannic  oxide.  Iron  and  zinc  are 
determined  in  the  filtrate  from  the  stannic  oxide. 

Preparation  of  a  Solution  for  Analysis. — Place  a 
weighed  quantity  (about  2  grams)  of  the  alloy  in  a 


BRONZE  225 

porcelain  basin  provided  with  a  cover-glass.  Add  30  c.c. 
of  water,  10  c.c.  of  concentrated  sulphuric  acid,  and 
10  c.c.  of  concentrated  nitric  acid.  Warm  until  the  alloy 
has  dissolved. 

Determination  of  Lead — Evaporate  the  solution  until 
all  the  nitric  acid  is  expelled  and  the  sulphuric  acid  fumes 
strongly.  Cool,  dilute  to  30  c.c.,  and  warm  until  the 
precipitated  sulphates  have  redissolved.  (There  is  no  risk 
of  precipitating  tin  at  this  stage  provided  the  solution 
contains  at  least  25  per  cent.,  by  volume,  of  sulphuric  acid.) 
Cool  the  solution,  and  dilute  with  cold  water  to  about  100 
c.c.  After  some  hours,  or  preferably  after  the  solution  has 
been  kept  overnight,  collect  the  lead  sulphate  by  filtration 
through  a  tared  Gooch  crucible.  Wash  with  dilute  sulphuric 
acid  (a  mixture  of  equal  volumes  of  the  bench  dilute  acid 
and  water)  and  treat  the  precipitate  as  described  on  p.  188. 
Weigh  the  PbSO4  obtained. 

Determination  of  Copper. — Determine  the  copper  in 
the  combined  filtrate  and  washings  electrolytically,  as 
described  on  p.  148  or  150.  (When  lead  is  absent  the 
original  solution  may  be  at  once  diluted  to  about  100  c.c. 
with  cold  water,  and  electrolysed.)  The  second  method,  with 
a  rotating  cathode,  is  to  be  preferred  as  the  more  expeditious. 
Whichever  method  is  used,  the  solution  must  not  be  heated. 

Determination  of  Tin. — After  removal  of  the  copper, 
dilute  the  solution  to  about  500  c.c.  Boil  gently  for  twenty 
to  thirty  minutes,  and  filter  through  a  "  blue  ribbon  "  filter 
paper,  using  slight  suction.  Wash  with  a  I  per  cent, 
sulphuric  acid  solution.  Dry  the  precipitate,  and  incinerate 
the  filter  paper  apart  from  the  precipitate  in  a  porcelain 
crucible.  Moisten  the  ash  with  a  drop  of  concentrated  nitric 
acid  in  order  to  reoxidise  any  reduced  oxide,  add  the 
precipitate,  ignite  strongly  until  of  constant  weight,  and 
weigh  the  SnO2. 

Determination  of  Iron. — Determine  the  iron  in  the 
filtrate  by  the  basic  acetate  method  (p.  165). 

Determination  of  Zinc. — After  removal  of  the  iron, 
determine  the  zinc  as  zinc  ammonium  phosphate  (p.  216). 


226        ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

ANALYSIS  OP  A  FUSIBLE  ALLOY. 

(Alloy  of  Bismuth,  Lead,  Tin,  and  Cadmium?) 

The  best  known  fusible  alloys  are  Newton's  alloy  (two 
parts  of  bismuth,  five  parts  of  lead,  and  three  parts  of  tin), 
Rose's  alloy  (two  parts  of  bismuth,  one  part  of  lead,  and  one 
part  of  tin),  and  Wood's  alloy  (four  parts  of  bismuth,  two 
parts  of  lead,  one  part  of  tin,  and  one  part  of  cadmium). 
The  method  described  below  is  applicable  to  any  of  these 
alloys. 

OUTLINE  OF  METHOD. — The  alloy  is  disintegrated  with  nitric  acid. 

The  insoluble  residue  of  impure  stannic  oxide  is  washed,  dried, 
ignited,  and  weighed  ;  it  is  then  fused  with  sodium  carbonate  and 
sulphur.  The  soluble  sodium  thiostannate  is  removed  by  extraction 
with  water.  The  insoluble  residue  of  lead  and  bismuth  sulphides  is 
dissolved  in  dilute  nitric  acid.  The  bismuth  is  determined  as  basic 
nitrate,  the  lead  as  sulphate,  and  the  necessary  corrections  for  the 
amounts  thus  found  applied  to  the  tin,  lead,  and  bismuth. 

The  filtrate  contains  bismuth,  lead,  and  cadmium,  as  nitrates. 
After  separation  of  the  bismuth  as  oxynitrate,  the  lead  is  determined 
as  sulphate.  The  cadmium  is  determined  either  electrolytically  or 
by  precipitation  as  sulphide. 

Place  a  weighed  portion  (about  06  gram)  of  the  finely 
divided  alloy  in  a  covered  porcelain  basin,  add  10  c.c.  of 
concentrated  nitric  acid,  and  proceed  as  in  the  case  of  solder 
(p.  223),  the  acid  being  kept  as  concentrated  as  possible. 
When  the  reaction  is  complete,  add  about  30  c.c.  of  water, 
and  boil  gently  for  a  few  minutes.  Filter,  and  wash  the 
insoluble  residue,  at  first  with  hot  dilute  nitric  acid,  and 
then  thoroughly  with  hot  water. 

Analysis  of  the  Insoluble  Residue. 

Determination  of  Tin. — The  insoluble  residue  consists  of 
stannic  oxide,  with  traces  of  bismuth  and  lead  oxides.  Dry, 
ignite,  and  weigh  the  impure  stannic  oxide  as  described  on 
p.  223.  Then  fuse  it  with  sodium  carbonate  and  sulphur, 
and,  after  cooling,  extract  the  sodium  thiostannate  with 
water.  (For  details  of  the  procedure,  see  under  analysis  of 
solder.)  Filter  through  a  tared  Gooch  crucible  and  wash 


FUSIBLE  ALLOY  227 

the  insoluble  residue  of  lead  and  bismuth  sulphides.  Then 
pour  boiling  dilute  nitric  acid  through  the  filter  until  the 
residue  has  dissolved.  (A  trace  of  the  lead  sulphide  may 
be  converted  into  insoluble  lead  sulphate ;  in  case  this  has 
happened,  use  the  same  crucible  for  the  filtration  of  the  lead 
sulphate  at  a  later  stage.)  Evaporate  almost  to  dryness, 
dilute  to  about  20  c.c,  and  add  very  dilute  ammonia  until 
the  solution  is  only  slightly  acid.  Filter,  wash  the  basic 
bismuth  nitrate  with  dilute  ammonium  nitrate  solution,  and 
convert  it  into  oxide  as  described  on  p.  173.  Weigh  the 
Bi2O3.  To  the  filtrate  from  the  basic  bismuth  nitrate,  add 
i  c.c.  of  concentrated  sulphuric  acid  and  evaporate  until 
dense  white  fumes  are  evolved.  Proceed  with  the  deter- 
mination of  the  lead  as  described  on  p.  188.  Weigh  the 
PbS04. 

Calculate  the  weight  of  PbO  corresponding  to  the  weight 
of  PbSO4  obtained.  Subtract  the  weights  of  lead  and  bismuth 
oxides  from  that  of  the  impure  stannic  oxide,  in  order  to 
obtain  the  weight  of  pure  stannic  oxide. 

Analysis  of  the  Soluble  Portion. 

Determination  of  Bismuth. — The  bismuth  may  be 
separated  as  basic  nitrate,  as  described  on  p.  172.  The 
following  modification  of  the  procedure  is,  however,  prefer- 
able, as  it  avoids  the  large  dilution. 

Evaporate  the  filtrate  and  washings  on  the  steam-bath 
until  the  solution  attains  a  syrupy  consistency.  Add  20  c.c. 
of  water,  stir  thoroughly,  and  again  evaporate.  Add  about 
100  c.c.  of  dilute  (2  grams  per  litre)  ammonium  nitrate 
solution,  and  keep  the  mixture  for  an  hour,  with  occasional 
vigorous  stirring,  before  filtering.  Wash  the  bismuth  oxy- 
nitrate  with  dilute  ammonium  nitrate  solution.  Convert  the 
bismuth  oxynitrate  into  oxide,  as  described  on  p.  173,  and 
weigh  as  Bi2O3.  Add  the  weight  of  bismuth  oxide  found  in 
the  crude  stannic  oxide. 

Determination  of  Lead — To  the  filtrate  from  the 
bismuth  oxynitrate,  add  5  c.c.  of  concentrated  sulphuric 
acid,  and  evaporate  until  dense  white  fumes  of  sulphuric 
acid  are  evolved.  Proceed  as  directed  on  p.  188,  but  wash 


228        ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

carefully  about  eight  times  with  dilute  sulphuric  acid  (a 
mixture  of  equal  volumes  of  dilute  acid  and  water)  before 
washing  with  alcohol ;  reject  the  alcohol  washings.  Weigh 
the  PbSO4,  and  add  the  amount  found  in  the  analysis  of 
the  insoluble  residue. 

Determination  of  Cadmium. — Determine  the  cadmium 
in  the  filtrate  either  electrolytically,  as  described  on  p.  149, 
or  by  precipitation  as  sulphide  and  conversion  into  sulphate. 
(For  details,  see  p.  174.) 

ANALYSIS  OP  A  LIMESTONE  OR  DOLOMITE. 

Limestone  consists  essentially  of  calcium  carbonate,  but 
may  contain  also  some  magnesium  carbonate.  If  the  pro- 
portion of  magnesium  carbonate  is  considerable,  the  rock  is 
called  a  dolomite.  The  usual  minor  constituents  are  iron, 
aluminium,  silica  (either  free  or  combined),  and  sometimes 
traces  of  carbonaceous  matter,  phosphate,  and  manganese. 
A  careful  qualitative  analysis  must  therefore  precede  the 
quantitative  analysis. 

In  the  following  description  of  the  analysis,  it  is  assumed 
that  a  dolomite  containing  magnesium  and  calcium  carbon- 
ates, with  small  quantities  of  iron,  aluminium,  silica  (or 
silicate),  and  phosphate  is  under  examination. 

OUTLINE  OF  METHOD. — The  silica  and  silicates  are  separated  from  the 
remainder  of  the  rock  by  treatment  with  hydrochloric  acid.  The 
soluble  and  insoluble  portions  are  examined  separately. 

In  the  soluble  portion  (i)  iron  and  aluminium,  together  with  any 
phosphate,  are  precipitated  by  ammonia  ;  (2)  in  the  filtrate,  calcium 
is  determined  by  precipitation  as  oxalate  ;  (3)  after  removal  of  the 
calcium,  the  magnesium  is  determined  as  phosphate. 

The  insoluble  residue,  after  ignition,  may  be  reported  simply 
as  silica  and  insoluble  silicates;  or,  after  fusion  with  sodium- 
carbonate,  the  silica  may  be  separated,  and  a  complete  analysis 
made. 

In  separate  portions  of  the  original  mineral,  carbonate,  water,  and 
phosphate  are  determined. 

Separation  into  Soluble  and  Insoluble  Portions. 

Reduce  about  10  grams  of  dolomite  to  a  fine  powder,  and 
place  the  powder  at  once  in  a  stoppered  weighing-bottle. 


LIMESTONE  OR  DOLOMITE  229 

Take  portions  of  this  powder  as  required,  the  weight  of  each 
portion  being  found  by  difference. 

Place  a  weighed  portion  (about  1-5  grams)  in  a  porcelain 
basin,  and  cover  the  basin  with  a  clock-glass  to  prevent  loss 
during  effervescence.  Moisten  the  powder  with  a  little  water, 
and,  by  means  of  a  pipette,  introduce  through  the  spout  of  the 
basin  10  c.c.  of  concentrated  hydrochloric  acid.  When  the 
action  has  almost  ceased,  rinse  the  cover-glass  and  the  side 
of  the  basin  with  water,  and  boil  for  a  few  minutes.  Again 
rinse  the  cover-glass  and  remove  it.  Evaporate  to  dryness — 
as  far  as  possible  on  the  steam-bath,  and  afterwards  on  a 
gently  heated  sand-bath.  Add  5  c.c.  of  concentrated  hydro- 
chloric acid  to  the  dry  mass,  and,  after  about  a  minute, 
dilute  with  about  10  c.c.  of  water;  warm  the  covered  basin 
on  the  steam-bath.  Filter  through  a  small  filter  paper ;  wash 
with  a  little  cold  water,  then  with  hot  dilute  hydrochloric 
acid,  and  finally  with  hot  water.  The  insoluble  residue  con- 
sists of  silica  and  insoluble  silicates ;  the  soluble  portion 
contains  the  main  portion  of  the  metallic  radicals  as 
chloride. 

Analysis  of  the  Soluble  Portion. 

For  the  most  exact  work,  the  trace  of  silica  present 
in  the  solution  must  be  removed  by  a  second  evaporation 
to  dryness  (cf.  p.  208),  but  for  all  ordinary  purposes  this  is 
unnecessary. 

Determination  of  Iron  and  Aluminium. — Add  about  5 
c.c.  of  concentrated  nitric  acid  in  order  to  oxidise  any  ferrous 
salt  and  to  form  ammonium  nitrate  when  ammonia  is  added. 
Heat  until  almost  boiling,  add  ammonia  until  slight  excess  is 
present,  and  boil  for  one  minute  (cf.  p.  127).  Filter,  and 
wash  three  times  with  hot  water,  without  attempting  to  bring 
all  the  precipitate  on  to  the  filter  paper.  The  precipitate  is 
mainly  ferric  and  aluminium  hydroxides  (together  with  any 
phosphate),  but  contains  also  traces  of  calcium  and  magnesium 
salts,  which  must  be  removed  by  reprecipitation.  Dissolve 
the  precipitate  which  remains  in  the  beaker  in  about  10  c.c. 
of  hot,  dilute  nitric  acid,  and  pour  the  hot  liquid  through  the 
filter  paper  in  order  to  dissolve  the  remainder  of  the  pre- 
cipitate. Wash  the  paper  a  few  times  with  hot  water,  and 


230       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

preserve  it  until  required  later.  To  the  filtrate,  add  about 
2  c.c.  of  concentrated  nitric  acid,  and  precipitate  the  iron  and 
aluminium  as  before.  Filter  through  the  same  filter  paper, 
combining  the  filtrate  with  that  from  the  first  precipitation, 
and  wash  thoroughly  with  hot  water.  Ignite  the  precipitate 
without  previous  drying :  heat  cautiously  at  first,  but  finally 
over  a  blowpipe  or  large  Meker  burner.  Cool,  and  weigh 
the  Fe2O3  and  A12O3.  It  is  seldom  necessary  to  separate 
the  iron  and  aluminium,  but,  if  required,  this  may  be  done 
as  described  on  p.  167. 

If  phosphate  is  present,  it  should  be  determined,  and  a 
correction  applied  to  the  alumina.  Calculate  the  phosphate 
as  P2O5,  arid  subtract  this  from  the  weight  of  alumina  as 
determined  above.  (The  phosphate  is  determined  in  a 
separate  portion  of  the  mineral.) 

Determination  of  Calcium. — Evaporate  the  combined 
filtrates  to  about  200  c.c.,  and  filter  through  a  small  filter 
paper.  The  trace  of  precipitate  which  almost  invariably 
separates  during  the  evaporation  consists  of  alumina  and 
calcium  carbonate.  Dissolve  it  in  a  little  dilute  nitric  acid, 
precipitate  the  alumina  with  ammonia,  filter,  and  add  the 
filtrate  to  the  main  solution.  Ignite  the  alumina,  and  add 
it  to  the  main  alumina  precipitate. 

Cover  the  solution  in  the  beaker  with  a  clock-glass, 
and  heat  until  boiling.  Remove  the  flame  and  at  once  add 
about  2  grams  of  solid  ammonium  oxalate.  Make  the 
solution  distinctly  alkaline  with  ammonia,  and  boil  gently 
until  the  precipitate  becomes  granular.  Keep  the  mixture 
for  one  hour,  decant  the  supernatant  liquid  through  a  filter, 
and  wash  three  or  four  times  with  hot  water,  retaining  the 
precipitate,  as  far  as  possible,  in  the  beaker.  Dissolve  the 
impure  calcium  oxalate  in  dilute  nitric  acid,  dilute  the  solu- 
tion to  about  200  c.c.,  heat  until  boiling,  and  reprecipitate 
the  calcium  oxalate  by  adding  about  2  c.c.  of  ammonium 
oxalate  solution  and  then  ammonia,  drop  by  drop,  until  the 
liquid  is  alkaline.  Boil  for  a  few  minutes  and  set  the  beaker 
aside  for  an  hour.  (If  magnesium  is  present  only  in  traces, 
one  precipitation  is  sufficient.)  Proceed  according  to  the 
directions  on  p.  143.  Combine  the  filtrates  from  the  two 
precipitations. 


LIMESTONE  OR  DOLOMITE  231 

Determination  of  Magnesium. — To  the  combined  filtrates, 
add  a  few  drops  of  methyl  orange,  and  then  add  hydrochloric 
acid  until  the  solution  is  almost  neutral.  Add  a  decided 
excess  of  microcosmic  salt  solution,  and  stir  for  a  few 
minutes.  Add  10  per  cent,  (by  volume)  of  concentrated 
ammonia,  and  set  the  solution  aside  for  at  least  twelve 
hours  (preferably  twenty-four  hours).  Filter,  and  wash 
with  dilute  ammonia.  Dissolve  the  precipitate  in  the 
minimum  amount  of  dilute  hydrochloric  acid.  Reprecipitate 
the  magnesium  ammonium  phosphate  by  adding  a  few  drops 
of  microcosmic  salt  solution,  and  then  ammonia  until  the 
liquid  is  decidedly  ammoniacal.  Stir  the  mixture  briskly, 
and  set  the  beaker  aside  for  a  few  hours.  Then  proceed 
according  to  the  directions  on  p.  136. 

Analysis  of  the  Insoluble  Portion. 

Incinerate  the  filter,  ignite  the  insoluble  residue  in  a 
platinum  crucible,  and  weigh.  If  the  insoluble  portion 
amounts  to  less  than  2  per  cent,  it  is  sufficient  for  most 
purposes  to  report  the  amount  of  "silica  and  insoluble 
silicates " ;  if  it  exceeds  that  amount,  analyse  it  according 
to  the  following  scheme. 

Determination  of  Silica. — Ignite  the  insoluble  residue 
with  the  filter  paper  in  a  platinum  crucible.  To  the  residue 
add  about  six  times  its  weight  of  anhydrous  sodium 
carbonate,  and  proceed  according  to  the  directions  given 
on  p.  232  for  the  determination  of  silica  in  an  insoluble 
silicate. 

The  filtrate  from  the  silica  may  contain  any  of  the 
constituents  found  in  the  soluble  portion.  It  may  be  added 
to  the  main  solution,  but  it  is  preferable  to  analyse  it  apart 
from,  although  in  the  same  manner  as,  the  main  solution. 

Determination  of  Other  Constituents. 

Separate  portions  of  the  original  mineraj  must  be  used 
for  the  determination  of  water,  carbonate,  and  phosphate. 

Water.— Determine  the  amount  of  water  by  Penfield's 
method,  using  about  2  grams  of  the  mineral  for  the 
determination.  For  details,  see  p.  215. 


232       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Carbonate. — Determine  the  carbonate  by  either  the 
direct  or  indirect  method  (pp.  175  and  179).  Use  about 
I  gram  of  the  dolomite  for  the  determination. 

Phosphate. — The  amount  present  is  often  so  small  as 
to  be  negligible.  If  it  is  to  be  determined,  2  to  5  grams  of 
the  dolomite  should  be  decomposed  with  dilute  nitric  acid, 
and  the  phosphate  determined  in  the  soluble  portion  by  the 
molybdate  method  (p.  197). 

ANALYSIS   OP   AN   INSOLUBLE   SILICATE. 

(Feldspar,   Clay,  Mica,  etc.) 

Most  of  the  natural  silicates,  such  as  clay,  feldspar,  garnet, 
and  mica  are  complex  alumino-silicates.  For  example,  ortho- 
clase  (potassium  feldspar)  may  be  represented  as  KAlSi3O8 ; 
anorthite  (calcium  feldspar)  as  CaAl2Si2O8;  albite  as 
NaAlSi3O8;  kaolinite  as  H4Al2Si2O9;  and  muscovite 
(common  or  potassium  mica)  as  H2KAl3(SiO4)3.  Pure 
forms  of  these  minerals  are,  however,  almost  unknown ; 
thus,  although  orthoclase  has  essentially  the  composition 
represented  by  KAlSi3O8,  in  almost  all  specimens  it  is 
found  that  the  potassium  is  to  some  extent  replaced  by 
sodium,  calcium,  and  magnesium,  whilst  the  aluminium  is 
usually  partially  replaced  by  iron.  The  analysis  of  a  silicate 
therefore  involves,  as  a  rule,  the  determination  of  silica, 
aluminium,  iron,  calcium,  magnesium,  sodium,  potassium, 
and,  in  some  cases,  carbonate  and  water. 

Full  information  on  this  important  branch  of  analytical 
chemistry,  with  details  for  the  analysis  of  more  complex 
silicates,  will  be  found  in  Hillebrand's  Analysis  of  Silicate 
and  Carbonate  Rocks  (Bulletin  422,  U.S.  Geological  Survey) ; 
the  practical  details  of  manipulation  are  minutely  described 
in  Washington's  Chemical  Analysis  of  Rocks  (Chapman 
and  Hall). 

OUTLINE  OF  METHOD. — A  portion  of  the  silicate  is  fused  with  sodium 
carbonate,  and  the  fused  mass  is  extracted  with  excess  of  acid.  The 
insoluble  residue  is  silica.  The  filtrate  contains  the  iron,  aluminium, 
calcium,  and  magnesium,  which  are  determined  as  follows :  Iron 
and  aluminium  are  precipitated  as  hydroxides,  the  amount  of  iron 
in  the  mixed  precipitate  being  determined  volumetrically.  After 


INSOLUBLE  SILICATE  233 

removal  of  the  iron  and  aluminium,  calcium  is  precipitated  as 
oxalate.  After  removal  of  the  calcium,  magnesium  is  determined 
as  phosphate. 

Separate  portions  of  the  silicate  are  used  for  the  determination 
of  (i)  sodium  and  potassium  by  the  Lawrence  Smith  method;  (2) 
water  j  and  (3)  carbonate. 

Break  the  minerals  into  small  pieces  on  a  clean  steel 
plate.  Take  about  10  grams  of  clean  pieces  of  the  mineral, 
and  crush  in  a  percussion  mortar  to  a  coarse  powder. 
Then  grind  to  a  fine  powder  in  an  agate  mortar.  The 
whole  analysis  is  facilitated  by  reducing  the  mineral  to  a 
fine  powder,  but  only  for  the  determination  of  the  alkalis 
is  it  essential  to  grind  to  the  finest  possible  powder.  The 
various  constituents  of  a  rock  often  differ  very  much  in 
hardness,  and  it  is  not  permissible  to  reject  the  portion 
which  offers  most  resistance  to  grinding,  as  this  portion 
probably  differs  in  composition  from  the  remainder.  When, 
therefore,  the  whole  of  the  sample  has  been  reduced  to  a 
fine  powder,  mix  it  thoroughly,  place  in  a  stoppered  bottle, 
and  use  portions  of  this  powder  for  each  of  the  following 
analyses. 

Determination  of  Silica,  Iron,  Aluminium,  Calcium, 
and  Magnesium. 

Fusion  with  Sodium  Carbonate. — Take  a  weighed 
portion  (0-9  to  i-i  gram)  of  the  powder,  and  fuse  it  with* 
6  grams  of  anhydrous  sodium  carbonate.  (For  details  of 
the  procedure,  see  p.  206.)  The  fusion  is  complete  when 
there  is  no  longer  any  evolution  of  gas  from  the  molten  mass, 
which,  however,  will  not  be  clear,  even  when  the  fusion 
is  complete,  since  the  carbonates  of  iron,  calcium,  and 
magnesium  are  not  dissolved  by  the  molten  mixture,  but 
remain  in  suspension  as  cloudy  masses. 

Determination  of  Silica. — Details  for  the  separation  of 
the  silica  after  the  fusion  are  given  on  p.  207. 

Determination  of  Iron,  Aluminium,  Calcium,  and 
Magnesium. — The  filtrate  from  the  silica  contains  these 
metals.  Determine  them  as  described  under  the  analysis 
of  dolomite  (p.  229). 


234        ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Determination  of  Sodium  and  Potassium. 

(Lawrence  Smith  Method?) 

OUTLINE  OF  METHOD. — The  silicate  is  decomposed  by  heating  with 
ammonium  chloride  and  calcium  carbonate.  On  extracting  the 
mass  with  water,  a  solution  of  the  chlorides  of  calcium,  sodium,  and 
potassium  is  obtained.  The  calcium  is  removed,  partly  as  carbonate 
and  the  remainder  as  oxalate.  The  sodium  and  potassium  are  then 
determined  in  the  usual  manner. 

The  ammonium  chloride  must  be  pure,  and  it  is  advisable 
to  sublime  a  sample  to  be  kept  for  this  determination  only. 
The  calcium  carbonate  must  always  be  purified  before  use. 
Dissolve  a  quantity  of  the  purest  obtainable  calcium  carbonate 
(or  pure  calcspar)  in  hydrochloric  acid,  precipitate  with  a 
freshly  prepared  solution  of  pure  ammonium  carbonate, 
filter,  and  wash  very  thoroughly  with  hot  water.  Even 
after  this  purification,  the  reagents  still  contain  traces  of 
alkalis,  derived  probably  from  the  vessels  employed.  A 
blank  experiment,  carried  out  with  the  same  quantities  of 
the  reagents  and  in  the  same  manner  as  in  the  actual 
analysis,  must  therefore  be  performed,  and  the  necessary 
correction  applied  in  subsequent  analyses.  If  the  weight 
of  alkali  chloride  from  0-5  gram  of  ammonium  chloride  and 
4  grams  of  calcium  carbonate  exceeds  2  mgrms.,  further 
purification  of  the  reagents  is  necessary. 

A  special  finger-shaped  platinum  crucible  is  most  suitable 
for  the  ignition,  but,  if  this  is  not  available,  an  ordinary,  30 
c.c.,  crucible  may  be  used.  Certain  precautions  are  necessary 
to  prevent  loss  by  volatilisation  of  the  alkali  chlorides.  The 
crucible  should  be  supported  on  a  perforated  silica  plate  so 
that  the  lowest  third  of  the  crucible,  but  not  more,  can  be 
heated  to  a  red  heat.  As  a  further  precaution  against  loss, 
use  as  a  lid  a  smaller,  closely  fitting,  platinum  crucible  filled 
with  water. 

Decomposition  of  the  Silicate. — Weigh,  by  difference, 
about  0-5  gram  of  the  powder  into  a  large  agate  mortar. 
Place  the  mortar  on  a  sheet  of  glazed  paper,  add  0-5  gram 
(roughly  weighed)  of  ammonium  chloride,  and  grind  the  two 
together  very  thoroughly.  For  a  successful  determination  of 
the  alkalis,  it  is  in  most  cases  essential  that  the  substance 


INSOLUBLE  SILICATE  235 

should  be  ground  to  the  finest  possible  powder.  (When  mica 
is  present,  it  cannot  be  reduced  to  a  very  fine  powder  on 
account  of  its  ready  cleavage  into  plates  and  the  flexibility 
of  these  plates ;  mica,  however,  is  more  readily  decomposed 
than  most  silicates,  and  less  thorough  grinding  therefore 
suffices  for  it.)  Weigh  approximately  4  grams  of  calcium 
carbonate,  place  most  of  this  in  the  mortar,  and  continue  the 
grinding  until  thorough  mixing  has  resulted.  Place  a  thin 
layer  of  calcium  carbonate  on  the  bottom  of  the  crucible  (in 
order  to  prevent  adhesion  of  the  mass  after  the  ignition), 
and,  with  the  aid  of  a  sheet  of  glazed  paper  and  a  brush, 
transfer  the  mixture  from  the  mortar  to  the  crucible,  using 
the  remainder  of  the  calcium  carbonate  for  "rinsing"  any 
traces  off  the  mortar  and  pestle. 

Support  the  crucible  on  a  perforated  silica  plate,  cover 
with  the  smaller  platinum  crucible  containing  water,  and 
heat  with  a  small  flame  for  about  ten  minutes.  When  the 
odour  of  ammonia  is  no  longer  perceptible,  increase  the 
flame  until  the  bottom  of  the  crucible  is  at  a  bright  red 
heat,  and  continue  the  heating  for  about  forty  minutes. 

Place  the  crucible  with  its  contents  in  a  porcelain  basin 
or  casserole,  and  extract  with  hot  water  until  the  whole  mass 
is  broken  up.  Filter,  and  wash  with  hot  water,  at  first  by 
decantation.  If  there  are  lumps,  break  them  up  with  a 
pestle  or  glass  rod,  and  wash  thoroughly  with  hot  water. 
If  there  is  any  sublimate  on  the  outside  of  the  crucible  used 
as  a  cover,  rinse  it  with  hot  water.  The  solution  contains 
all  the  sodium  and  potassium  as  chloride,  together  with  some 
calcium  chloride. 

Removal  of  Calcium. — To  the  hot  solution  in  a  porcelain 
basin,  add  10  c.c.  of  ammonia  and  a  slight  excess  of  a  freshly 
prepared  solution  of  ammonium  carbonate.  Filter  into  a 
platinum  basin.  Dissolve  the  precipitate  in  the  minimum 
amount  of  hydrochloric  acid,  reprecipitate  the  calcium  with 
ammonium  carbonate,  filter,  and  wash.  Add  the  second 
filtrate  to  the  first,  evaporate  to  complete  dryness,  and  drive 
off  the  ammonium  salts  by  gentle  ignition.  (See  p.  204  for 
certain  precautions  during  this  operation.)  The  residue 
always  contains  a  trace  of  calcium  which  is  removed  as 
follows: — Dissolve  the  residue  in  about  10  c.c.  of  water;  add 


236       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

two  drops  of  ammonia  and  I  c.c.  of  ammonium  oxalate. 
Filter  through  a  very  small  filter  paper  into  a  platinum  dish, 
wash  thoroughly  with  warm  water  containing  a  little  ammonia, 
evaporate  the  filtrate  to  dryness,  and  ignite  gently.  Moisten 
the  residue  with  hydrochloric  acid  in  order  to  convert  any 
carbonate  into  chloride,  dry,  and  ignite  cautiously.  (Care 
must  be  taken  not  to  heat  sufficiently  strongly  to  volatilise 
any  alkali  chloride.) 

Determination  of  Sodium  and  Potassium. — The  weight 
of  the  residue  gives  the  weight  of  the  mixed  sodium  and 
potassium  chlorides.  Determine  the  potassium,  either  as 
perchlorate  or  as  chloroplatinate,  and  find  the  amount  of 
sodium  by  difference  (p.  204). 

Note. — If  the  mixed  chlorides  do  not  dissolve  completely 
in  water,  collect  the  insoluble  residue  on  a  small  filter  paper, 
wash  with  hot  water,  ignite,  and  weigh.  If  the  weight  is  less 
than  i  mgrm.,  subtract  it  from  that  of  the  mixed  chlorides ; 
if  more  than  i  mgrm.,  reject  the  analysis. 

Determination  of  Water  and  Carbon  Dioxide. 

Use  separate  portions  of  the  powdered  mineral  for  (i) 
the  determination  of  water,  as  described  on  p.  215  ;  and  (2) 
the  determination  of  carbonate,  as  described  on  p.  175. 
Many  silicates  contain  no  carbonate,  but  it  is  often  found 
in  clays. 

ANALYSIS   OP   A   GLASS. 

Ordinary  "  soft "  glass,  used  for  window  glass,  bottles,  etc., 
is  essentially  a  sodium-calcium  silicate,  whilst  in  "hard" 
glass  (Bohemian  glass)  the  sodium  is  replaced  by  potassium. 
Flint  glass  is  a  potassium-lead  silicate.  Traces  of  iron, 
aluminium,  and  manganese  are  usually  present. 

Glass  for  special  purposes  sometimes  contains  borate. 
Oxides  of  copper,  iron,  cobalt,  and  manganese  are  used  in 
the  preparation  of  coloured  glasses.  Bone  ash,  cryolite,  or 
fluorspar  is  added  to  common  glass  when  it  is  desired  to 
render  it  opaque. 

The  common  constituents  of  glass  are  therefore  lead, 
calcium,  sodium,  potassium,  and  silica,  with  traces  of  iron 


GLASS  237 

aluminium,  and  manganese.  The  analysis  of  a  glass  is 
therefore  carried  out  according  to  the  ordinary  procedure 
for  an  insoluble  silicate,  but  when  lead  and  manganese  are 
present  the  method  must  be  modified  as  follows : — 

OUTLINE  OF  METHOD. — The  glass  is  fused  with  sodium  carbonate,  and 
the  fused  mass  is  extracted  with  hydrochloric  acid.  Silica  is  deter- 
mined in  the  insoluble  residue.  In  the  filtrate  from  the  silica,  the 
lead  is  precipitated  as  sulphide.  After  removal  of  lead,  iron  and 
aluminium  are  precipitated  as  basic  acetates,  and  the  manganese  in 
the  filtrate  is  precipitated  as  sulphide.  After  removal  of  manganese, 
calcium  is  precipitated  as  oxalate. 

A  separate  portion  of  the  glass  is  decomposed  with  hydrofluoric 
acid.  The  sodium  and  potassium  are  thus  obtained  as  soluble  salts, 
and  are  determined  in  the  usual  manner  after  removal  of  all  other 
metals. 

Fusion  with  Sodium  Carbonate  and  Determination  of 
Silica. — Fuse  a  weighed  portion  (about  I  gram)  of  the  glass 
with  sodium  carbonate,  as  described  on  p.  206.  Wash  the 
insoluble  residue  very  thoroughly  with  hot  dilute  hydro- 
chloric acid,  as  the  lead  chloride  is  somewhat  difficult  to 
remove.  Test  the  purity  of  the  silica  in  the  usual 
manner. 

Determination  of  Lead. — The  precipitation  of  lead  as 
sulphide  must  be  performed  in  acid  solution,  but,  as  no  lead 
sulphide  is  precipitated  if  the  amount  of  hydrochloric  acid 
present  exceeds  3  per  cent.,  careful  adjustment  of  the  acidity 
is  essential. 

Evaporate  the  solution,  after  the  removal  of  the  silica, 
almost  to  dryness,  add  10  c.c.  of  concentrated  hydrochloric 
acid,  and  dilute  to  200  c.c.  Saturate  the  solution  with 
hydrogen  sulphide,  filter,  and  wash  with  hydrogen  sulphide 
solution  acidified  with  a  little  hydrochloric  acid.  Dry  the 
filter  and  contents,  and  incinerate  the  paper  apart  from  the 
precipitate.  Moisten  the  ash  and  the  precipitate  with  con- 
centrated nitric  acid,  add  one  drop  of  concentrated  sulphuric 
acid,  and  warm  gently  until  dry.  Repeat  this  operation  until 
the  mass  is  perfectly  white ;  then  heat  more  strongly,  and 
weigh  as  PbSO4  (cf.  p.  188). 

Determination  of  Iron  and  Aluminium. — Boil  the  filtrate 
from  the  lead  sulphide  until  it  ceases  to  smell  of  hydrogen 
sulphide,  and  then  precipitate  the  iron  and  aluminium  as 


238       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

basic  acetates  (p.  165).  Determine  the  iron  and  aluminium 
in  the  precipitate. 

Determination  of  Manganese. — Combine  the  filtrates 
from  the  basic  acetate  precipitations,  acidify  with  hydro- 
chloric acid,  and  evaporate  in  a  porcelain  basin  until  the 
volume  is  reduced  to  about  50  c.c.  Transfer  the  solution  to  a 
100  c.c.  conical  flask,  and  add  5  grams  of  ammonium  chloride. 
Add  i  c.c.  of  methyl  red  and  then  ammonia  until  the 
solution  is  slightly  alkaline.  To  the  cold  solution,  add  a 
slight  excess  of  freshly  prepared,  colourless,  ammonium 
sulphide  solution,  and  almost  fill  the  flask  with  cold 
carbonate-free  (recently  boiled)  water.  Cork  the  flask,  and 
set  it  aside  for  twenty-four  hours.  The  precipitate  is  difficult 
to  filter,  and  should  therefore  be  washed,  as  far  as  possible, 
by  decantation.  Wash  with  a  solution  prepared  by  dissolving 
5  grams  of  ammonium  chloride  in  100  c.c.  of  water  and 
adding  I  c.c.  of  colourless  ammonium  sulphide  solution. 

The  weight  of  the  precipitate  usually  amounts  only  to  a 
few  milligrams.  The  manganese  in  it  is  best  determined 
colorimetrically,  as  described  on  p.  162. 

Determination  of  Calcium. — Determine  the  calcium  in 
the  filtrate  from  the  manganese  sulphide,  by  precipitation  as 
calcium  oxalate,  with  subsequent  conversion  into  oxide  or 
carbonate  (p.  143). 

Determination  of  Sodium  and  Potassium. 

OUTLINE  OF  METHOD.— The  glass  is  decomposed  and  the  silica 
volatilised  by  treatment  with  hydrofluoric  acid.  The  metals 
present  are  thus  obtained  as  fluorides,  which  are  converted  into 
sulphates  by  treatment  with  sulphuric  acid.  All  metals  other  than 
the  alkalis  are  then  removed,  and  the  sodium  and  potassium  are 
determined  in  the  usual  manner. 

Place  a  weighed  portion  (about  I  gram)  of  the  finely 
powdered  glass  in  a  platinum  basin,  moisten  with  water, 
and  add  about  10  c.c.  of  pure  hydrofluoric  acid.  (The 
purity  of  the  hydrofluoric  acid  should  be  tested  by 
evaporating  10  c.c.  to  dryness ;  no  weighable  residue  should 
be  obtained.)  Mix  the  powder  thoroughly  with  the  acid  by 
means  of  a  platinum  spatula  or  stout  platinum  wire.  Cover 
loosely  with  a  larger  platinum  basin  and  set  aside  for  twelve 


IRON  PYRITES  239 

hours.  Then  add  a  further  5  c.c.  of  hydrofluoric  acid,  and 
evaporate  to  dryness  on  the  steam-bath. 

To  the  residue  of  fluorides,  add  about  2  c.c.  of  water 
and  i  c.c.  of  concentrated  sulphuric  acid,  and  cover  the  basin 
with  a  larger  platinum  basin  held  by  means  of  platinum  wires 
so  that  it  does  not  completely  close  the  lower  basin. 
Evaporate  as  far  as  possible  on  the  steam-bath,  then  remove 
to  a  sand-bath,  and  heat  very  gently  until  no  more  fumes  of 
sulphuric  acid  are  evolved.  Moisten  the  residue  with  con- 
centrated hydrochloric  acid.  Remove  the  cover,  and  rinse 
it  into  the  basin  with  hot  water.  Transfer  the  solution, 
together  with  any  undissolved  lead  sulphate,  to  a  400  c.c. 
glass  beaker,  washing  the  platinum  basin  thoroughly  with 
hot  water. 

Dilute  to  about  150  c.c.,  and  heat  until  boiling.  To  the 
hot  solution,  add  a  slight  excess  of  a  hot,  saturated  solution 
of  barium  hydroxide.  Keep  the  mixture  hot  for  about  thirty 
minutes,  then  filter  from  the  precipitate  of  barium  sulphate, 
aluminium  hydroxide,  etc.,  and  wash  with  hot  water.  The 
filtrate  contains  calcium,  barium,  sodium,  and  potassium. 
Remove  the  calcium  and  barium,  and  determine  the  sodium 
and  potassium,  as  described  on  p.  235.  The  barium  is 
removed,  together  with  the  calcium,  by  precipitation  with 
ammonium  carbonate,  as  described  under  "  Removal  of 
Calcium." 

ANALYSIS  OP  IRON  PYRITES. 

Iron  pyrites  (pyrite)  consists  essentially  of  sulphide  of 
iron,  FeS2,  and  in  a  good  specimen  the  amount  of  other 
elements  is  very  small.  In  many  specimens,  the  iron  is 
partially  replaced  by  copper,  and  traces  of  arsenic,  cobalt, 
and  nickel  are  often  present.  Most  specimens  contain  also 
a  certain  amount  of  gangue — consisting  of  enclosed  or 
adhering  particles  of  sand  or  other  siliceous  matter. 

The  analysis  of  iron  pyrites  therefore  involves,  as  a  rule, 
the  determination  of  "gangue,"  copper,  iron,  and  sulphur. 
It  is  sometimes  necessary  to  determine  also  traces  of  arsenic, 
nickel,  and  cobalt ;  in  such  cases,  the  method  described  below 
can  be  readily  modified  to  include  the  determination  of  these 
elements. 


240        ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Iron  pyrites  may  also  be  analysed,  by  the  method 
described  on  p.  241,  for  copper  pyrites,  but  the  following 
method  is  probably  preferable. 

OUTLINE  OF  METHOD.— The  finely  divided  mineral  is  oxidised  by  means 
of  potassium  chlorate  and  hydrochloric  acid.  The  insoluble  residue 
is  separated  by  filtration,  ignited,  and  weighed.  The  sulphur  has 
been  completely  oxidised  to  sulphate,  which  is  determined  as 
follows  : — The  iron  is  precipitated  by  addition  of  ammonia,  and, 
without  filtering,  the  sulphate  is  precipitated  as  barium  sulphate. 
The  solution  is  then  acidified  with  hydrochloric  acid,  in  order  to  re- 
dissolve  the  ferric  hydroxide,  and  the  barium  sulphate  is  collected 
and  weighed. 

The  copper  is  precipitated  from  the  filtrate  as  sulphide,  and  the 
iron  is  then  determined  either  volumetrically  or  by  precipitation  as 
hydroxide. 

Decomposition  of  the  Pyrites. — Place  about  0-3  gram  of 
the  finely  powdered  pyrites  in  a  dry  200  c.c.  conical  flask, 
mix  with  2  grams  of  powdered  potassium  chlorate,  moisten 
with  3  c.c.  of  water,  and  cool  the  flask  and  contents  in  ice. 
Add  20  c.c.  of  concentrated  hydrochloric  acid,  previously 
cooled  in  ice,  and  keep  the  flask  in  the  ice  for  twenty  to 
thirty  minutes,  with  occasional  gentle  shaking.  Remove  the 
flask  from  the  ice,  so  that  the  temperature  will  rise  slowly. 
After  about  thirty  minutes,  warm  the  flask  momentarily  on 
the  steam-bath,  repeating  this  at  intervals  with  gentle  shaking, 
until  the  pyrites  has  entirely  disappeared.  The  decomposi- 
tion should  proceed  without  the  separation  of  sulphur,  the 
oxidation  of  which,  if  it  is  in  the  form  of  lumps  or  liquid 
globules,  is  exceedingly  tedious. 

Transfer  the  solution,  together  with  any  insoluble  gangue, 
to  a  porcelain  basin,  and  evaporate  to  dryness  on  the  steam- 
bath.  Cover  the  dry  residue  with  5  c.c.  of  concentrated 
hydrochloric  acid,  and,  after  five  minutes,  warm,  and  add 
about  30  c.c.  of  water.  Filter  into  a  400  c.c.  beaker,  and 
wash  the  residue  with  hot  dilute  hydrochloric  acid  and  then 
thoroughly  with  hot  water.  Incinerate  the  filter,  ignite  the 
residue  strongly,  and  weigh.  The  residue  consists,  as  a  rule, 
of  silica  or  insoluble  silicates,  and  may  usually  be  reported 
as  "  insoluble  residue"  or  "gangue"  ;  if,  however,  an  analysis 
of  it  is  required,  proceed  as  directed  on  p.  232. 

Determination  of  Sulphur — Dilute  the  solution  to  about 


COPPER  PYRITES  241 

200  c.c.  with  cold  water,  and  add  ammonia  in  slight  excess  to 
the  cold  solution  to  precipitate  the  iron.  Heat  the  solution 
until  boiling,  and  precipitate  the  sulphate  with  a  boiling 
solution  of  barium  chloride,  as  described  on  p.  131.  In 
order  to  estimate  the  quantity  of  reagent  required,  assume 
that  the  mineral  is  pure  pyrites,  and  use  5  per  cent, 
more  than  the  calculated  amount,  dissolved  in  25  c.c.  of 
water. 

Add  dilute  hydrochloric  acid  to  the  mixture  in  which  the 
barium  sulphate  and  ferric  hydroxide  are  suspended,  and, 
with  frequent  stirring,  make  sure  that  the  latter  has  com- 
pletely dissolved.  Cover  the  beaker  and  set  it  aside  for  at 
least  six  hours.  The  barium  sulphate  should  appear  per- 
fectly white.  Filter,  wash,  and  weigh  the  barium  sulphate. 

Note.^-li  barium  sulphate  is  precipitated  from  an  acid 
solution  containing  ferric  salts,  it  always  carries  down  part 
of  the  iron,  and  this  cannot  be  removed  by  washing.  By 
adopting  the  above  procedure,  the  sulphate  is  precipitated 
from  a  solution  which  contains  no  iron.  The  ignited 
barium  sulphate  should  be  pure  white,  and,  after  warming 
with  a  few  drops  of  concentrated  hydrochloric  acid,  should 
give  no  blue  coloration  with  potassium  ferrocyanide. 

Determination  of  Copper. — In  the  filtrate  from  the 
barium  sulphate  determine  the  copper  as  sulphide,  as 
described  on  p.  141.  The  solution  from  which  the  copper 
is  precipitated  by  hydrogen  sulphide  should  contain 
sufficient  free  hydrochloric  acid  to  prevent  co-precipitation 
of  nickel,  in  case  any  is  present. 

Determination  of  Iron. — After  removal  of  the  copper, 
determine  the  iron  either  volumetrically  or  by  precipitation 
as  hydroxide.  If  the  iron  is  to  be  determined  by  the  latter 
method,  evaporate  the  solution  in  an  open  basin  until  it 
ceases  to  smell  of  hydrogen  sulphide ;  then  proceed  as 
directed  on  p.  127. 

ANALYSIS  OF  COPPER  PYRITES. 

Copper  pyrites  (chalcopyrite)  consists  essentially  of  a 
copper-iron  sulphide,  CuFeS2,  and  in  a  good  specimen  of 
the  mineral  the  amount  of  other  elements  is  negligibly 

Q 


242       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

small.     Many  specimens  contain  traces  of  silver,  and  more 
or  less  gangue  is  usually  present. 

The  analysis  of  copper  pyrites  therefore  presents  a  problem 
very  similar  to  that  of  iron  pyrites.  The  same  methods 
are  applicable  to  both,  and  copper  pyrites  may  be  analysed 
by  the  method  described  on  p.  239,  or  by  the  method 
described  below.  Both  methods  for  the  determination  of 
sulphur  yield  satisfactory  results,  but  the  following  method  is 
better  adapted  for  the  determination  of  the  copper  and  iron 
in  copper  pyrites. 

OUTLINE  OF  METHOD. — In  one  portion  of  the  mineral,  the  sulphur  is 
completely  oxidised  to  sulphate  by  heating  with  sodium  peroxide. 
The  sulphate  is  then  determined  as  barium  sulphate. 

Another  portion  of  the  mineral  is  decomposed  with  dilute  nitric 
and  sulphuric  acids.  The  insoluble  residue  of  silica,  etc.,  is  separated 
by  filtration,  and  the  copper  in  the  solution  precipitated  by  means  of 
aluminium.  The  precipitated  copper  is  collected  on  a  Gooch  filter, 
washed,  dried,  and  weighed.  The  iron  in  the  filtrate  is  determined 
volumetrically. 

Determination  of  Sulphur. 

Place  about  5  grams  of  sodium  peroxide  in  a  clean  nickel 
or  iron  crucible,  add  a  weighed  quantity  (about  0-3  gram)  of 
finely  powdered  pyrites,  and  mix  thoroughly  by  means  of  a 
glass  rod,  avoiding  any  unnecessary  friction.  Cover  with  a 
layer  of  sodium  peroxide  (about  2  grams),  and  heat  very 
cautiously  with  a  small  flame  held  several  inches  below  the 
crucible,  until  it  is  evident  that  the  reaction  between  the 
sulphide  and  the  peroxide  has  started.  Heat  finally  until 
the  peroxide  shows  signs  of  fusing  at  the  edges  of  the 
crucible,  but  not  past  this  stage. 

Cool,  place  the  crucible  in  a  deep  porcelain  basin  or 
casserole,  and  extract  with  hot  water  until  the  mass  is 
completely  detached  from  the  crucible.  Transfer  the  liquid 
and  insoluble  residue  to  a  400  c.c.  beaker,  and  dilute  to 
about  200  c.c.  with  boiling  water.  The  insoluble  portion 
should  be  quite  free  from  gritty  particles.  Without  filtering, 
add  to  the  hot  solution  a  boiling  solution  of  barium  chloride 
in  slight  excess  (cf.  p.  241).  Then  add  more  than  sufficient 
hydrochloric  acid  to  dissolve  the  copper  and  iron  oxides, 
cover  the  beaker,  and  place  it  on  the  steam-bath  for  an 


COPPER  PYRITES  243 

hour.  Set  the  beaker  aside  for  at  least  six  hours  and  then 
collect  the  barium  sulphate  on  a  filter.  Wash,  ignite,  and 
weigh  the  barium  sulphate.  After  ignition,  it  should  still 
be  perfectly  white. 

Determination  of  Copper  and  Iron. 

Place  a  weighed  portion  (about  0-4  gram)  of  the  finely 
powdered  pyrites  in  a  porcelain  basin  or  casserole  provided 
with  a  cover-glass.  Add  20  c.c.  of  water,  5  c.c.  of  con- 
centrated nitric  acid,  and  5  c.c.  of  concentrated  sulphuric 
acid.  Warm  very  cautiously,  adjusting  the  heating  so  that 
there  is  a  vigorous,  but  not  turbulent,  evolution  of  brown 
fumes.  If  this  operation  is  properly  conducted,  there  will  be 
no  residue  of  sulphur  after  about  ten  minutes'  treatment; 
if  any  sulphur  remains,  it  must  be  brought  into  solution  by 
continued  gentle  boiling,  with  occasional  addition  of  a  few 
drops  of  concentrated  nitric  acid.  (If  the  sulphur  collects 
into  a  single  large  bead,  it  is  more  quickly  dissolved  by 
removing  it  with  a  glass  rod,  and  boiling  it  with  a  few  drops 
of  concentrated  sulphuric  acid.  Both  solutions  must  be 
cooled  before  this  is  rinsed  back  into  the  main  solution.) 

When  all  the  sulphur  is  oxidised,  add  a  further  5  c.c.  of 
concentrated  sulphuric  acid,  and  evaporate  until  the  solution 
is  fuming  strongly ;  cool,  and  dilute  the  solution  to  about 
40  c.c.  Heat  the  solution,  and  keep  it  hot  until  any 
anhydrous  ferric  sulphate  has  dissolved.  Filter,  wash  with 
cold  and  then  with  hot  water,  ignite  the  insoluble  residue, 
and  weigh. 

Determination  of  Copper. — Cut  some  sheet  aluminium 
into  pieces  about  2  by  4  cm.,  and  bend  each  piece  at  right 
angles  across  the  middle.  Place  four  or  five  of  these  pieces 
of  aluminium  in  the  solution,  cover  the  beaker,  and  heat 
gently  until  the  solution  is  colourless.  The  copper  is 
completely  precipitated  as  metallic  copper,  and  it  adheres 
so  loosely  to  the  aluminium  that  it  may  be  readily  removed 
by  a  jet  of  water.  Collect  the  copper  in  a  tared  Gooch 
filter,  wash  quickly  but  thoroughly  with  cold  water,  then 
three  times  with  alcohol,  and  dry  in  the  steam-oven  for  not 
more  than  ten  minutes.  (Prolonged  heating  may  cause 
oxidation  of  the  copper.)  Cool,  and  weigh  the  copper. 


244        ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Determination  of  Iron. — During  the  precipitation  of  the 
copper,  the  iron  is  reduced  to  ferrous  sulphate,  and  may 
therefore  be  titrated  at  once  in  the  filtrate  with  decinormal 
permanganate  or  dichromate. 

ANALYSIS   OP   GALENA. 

Galena  consists  mainly  of  lead  sulphide,  with  traces  of 
silver,  antimony,  copper,  arsenic,  iron,  manganese,  and  zinc. 
It  is  usually  admixed  with  more  or  less  silica  or  silicates. 
The  complete  analysis  of  galena  is  obviously  a  complex 
problem.  The  following  description  gives  details  for  the 
determination  of  the  lead,  sulphur,  and  "  insoluble  residue  " 
only;  as  the  lead  sulphide  comprises  96  to  99  per  cent,  of 
a  good  specimen  of  the  mineral,  it  is  often  sufficient  to 
determine  these  constituents  only. 

OUTLINE  OF  METHOD.— The  mineral  is  treated  with  nitric  acid  and 
bromine  until  all  the  sulphur  is  oxidised  to  sulphate.  The  lead 
sulphate  is  dissolved  in  concentrated  hydrochloric  acid,  filtered  from 
the  insoluble  residue,  and  the  lead  precipitated  by  addition  of 
hydrogen  peroxide.  The  sulphate  in  the  filtrate  is  determined  as 
barium  sulphate.  The  filtrate  from  the  barium  sulphate  contains 
the  copper,  iron,  etc.,  and  may  be  further  examined,  if  desired. 

Procedure. 

Weigh  accurately  07  to  08  gram  of  the  finely  powdered 
mineral.  Place  the  weighed  sample  in  a  porcelain  basin  or 
casserole,  and  moisten  with  dilute  nitric  acid.  After  a  few 
minutes,  add  10  c.c.  of  concentrated  nitric  acid,  and  heat 
the  covered  basin  on  the  steam-bath  for  twenty  minutes. 
Evaporate  nearly  to  dryness,  and  add  more  nitric  acid  and 
a  few  drops  of  bromine.  Continue  this  treatment  until  all 
the  sulphur  is  oxidised  to  sulphate,  i.e.,  until  the  residue 
is  white. 

Evaporate  to  dryness,  add  5  c.c.  of  concentrated  nitric 
acid,  and  again  evaporate  to  dryness.  Add  a  further  5  c.c. 
of  nitric  acid,  and  evaporate  for  the  third  time  to  dryness. 
This  treatment  is  necessary  in  order  to  destroy  bromate. 

To  the  residue,  add  60  c.c.  of  water  and  20  c.c.  of  con- 
centrated hydrochloric  acid,  and  warm  until  all  the  lead 
sulphate  has  dissolved.  Filter,  wash  with  a  little  hot  dilute 


ZINC  BLENDE  245 

hydrochloric  acid,  and  then  thoroughly  with  hot  water,  and 
ignite  paper  and  precipitate  without  preliminary  drying. 
Weigh  the  residue  of  silica  and  insoluble  silicates.  It  is 
usually  sufficient  to  report  this  portion  as  "  insoluble 
residue,"  since  it  is  not  an  essential  constituent  of  the 
mineral ;  if  a  further  examination  is  necessary,  proceed 
according  to  the  instructions  for  the  analysis  of  an  insoluble 
silicate  (p.  232). 

Determination  of  Lead. — Heat  the  filtrate  until  the 
precipitate  which  separates  on  cooling  has  redissolved. 
Add  to  the  hot  solution  a  mixture  of  100  c.c.  of  (3  per  cent.) 
hydrogen  peroxide,  50  c.c.  of  concentrated  ammonia,  and 
25  c.c.  of  water.  The  lead  separates  as  a  bright-yellow 
crystalline  precipitate  (probably  hydrated  lead  peroxide). 
Keep  for  some  hours,  with  occasional  stirring,  before 
filtration.  Filter,  wash  with  cold  water,  dry,  and  incinerate 
the  filter  paper  apart  from  the  precipitate  in  a  porcelain 
crucible.  Moisten  the  ash  with  a  drop  or  two  of  concen- 
trated nitric  acid,  in  order  to  oxidise  the  traces  of  metallic 
lead  which  are  formed  during  the  incineration.  Add 
the  precipitate,  ignite  at  a  low  red  heat,  and  weigh  as 
PbO. 

Determination  of  Sulphur. — Evaporate  the  filtrate  until 
it  ceases  to  smell  of  ammonia.  In  order  to  decompose  any 
persulphate  which  may  be  present,  add  5  c.c.  of  concentrated 
hydrochloric  acid  and  5  c.c.  of  alcohol.  Heat  for  a  few 
minutes,  and  determine  the  sulphate  according  to  the 
instructions  on  p.  131. 

Determination  of  Other  Metals. — The  filtrate  from  the 
barium  sulphate  contains  the  other  metals  which  were 
present  in  the  original  ore,  and  may  be  further  examined, 
if  desired. 

ANALYSIS  OP  ZINC  BLENDE. 

Zinc  blende  consists  essentially  of  zinc  sulphide,  but 
usually  contains  also  traces  of  carbonate,  cadmium,  copper, 
lead,  iron,  and  manganese.  With  most  samples  there  is 
associated  a  certain  amount  of  adhering  silicious  matter. 

OUTLINE  OF  METHOD. — One  portion  of  the  blende  is  decomposed  with 
hydrochloric  acid,  and  the  insoluble  residue  is  separated.     Lead  is 


246       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

determined  in  the  filtrate  as  sulphate.  After  removal  of  the  lead, 
the  copper  is  precipitated  by  means  of  hydrogen  sulphide  from  a 
strongly  acid  solution  ;  the  cadmium  is  then  precipitated  in  a 
similar  manner  from  a  slightly  acid  solution.  After  removal  of 
the  copper  and  cadmium,  the  iron  is  precipitated  by  means  of 
"  cupferron,"  and  the  manganese  is  then  separated  as  dioxide.  In 
the  filtrate,  zinc  is  determined  as  zinc  ammonium  phosphate. 

Sulphide  and  carbonate  are  determined  in  separate  portions  of 
the  mineral. 

One  point  in  connection  with  this  analysis  is  worthy  of 
special  mention,  namely,  the  difficulty  of  separating  cadmium 
and  zinc.  In  order  to  prevent  precipitation  of  zinc  by 
hydrogen  sulphide  when  other  metals  are  being  precipitated, 
it  is  necessary  to  make  the  solution  strongly  acid ;  but  if  the 
solution  is  made  strongly  acid  no  cadmium  is  precipitated. 
By  adopting  the  procedure  described  below,  the  contamina- 
tion of  the  cadmium  sulphide  with  zinc  sulphide  is  greatly 
reduced,  and,  as  only  a  small  quantity  of  cadmium  sulphide 
is  obtained  in  the  analysis  of  zinc  blende,  the  error  is 
negligible.  In  the  analysis  of  an  ore  rich  in  cadmium, 
repeated  precipitations  are  necessary  in  order  to  free  the 
cadmium  sulphide  from  zinc  sulphide. 

Separation  into  Soluble  and  Insoluble  Portions. — 
Introduce  a  weighed  portion  (about  i  gram)  of  the  finely 
powdered  zinc  blende  into  a  conical  flask,  and  moisten  the 
powder  with  water.  Add  about  20  c.c.  of  concentrated 
hydrochloric  acid,  and  close  the  flask  loosely  with  a  small 
funnel  or  glass  bulb.  Warm  on  the  steam-bath  until  there 
is  no  further  action,  and  then  add  from  time  to  time  a  few 
drops  of  concentrated  nitric  acid.  When  the  residue  is 
white  and  when  any  sulphur  has  been  brought  into  solution, 
add  a  further  10  c.c.  of  concentrated  hydrochloric  acid,  and 
boil  for  five  minutes.  Filter  the  hot  solution  through  a  small 
filter  paper  and  wash  with  a  boiling  solution  of  hydrochloric 
acid  (half  water  and  half  concentrated  acid)  in  order  to 
dissolve  any  lead  sulphate.  Wash  the  residue  with  hot  water, 
and  ignite  it.  Weigh,  and  report  the  result  as  "insoluble 
residue." 

Determination  of  Lead. — Evaporate  the  solution  to 
about  10  c.c.,  cool,  add  5  c.c.  of  concentrated  sulphuric  acid, 
and  evaporate  on  a  gently  heated  sand-bath  until  dense 


ZINC  BLENDE  247 

fumes  of  sulphuric  acid  are  evolved.  Cool,  dilute  to  about 
100  c.c.,  and  collect  the  precipitate  of  lead  sulphate  as 
described  on  p.  187.  The  precipitate  must  be  thoroughly 
washed  with  dilute  (2N)  sulphuric  acid  before  washing  with 
alcohol. 

Determination  of  Copper. — Evaporate  the  filtrate  and 
washings  from  the  lead  sulphate  until  it  begins  to  evolve 
fumes  of  sulphuric  acid.  Cool,  add  20  c.c.  of  water  and 
20  c.c.  of  concentrated  hydrochloric  acid,  and  saturate  the 
solution  with  hydrogen  sulphide.  Filter,  and  wash  with  a 
mixture  of  equal  volumes  of  concentrated  hydrochloric  acid 
and  saturated  hydrogen  sulphide  solution,  observing  the 
precautions  against  oxidation  mentioned  on  p.  142.  (When 
precipitated  from  this  strongly  acid  solution,  the  copper 
sulphide  is  practically  free  from  cadmium  and  zinc.) 
Convert  the  cupric  sulphide  into  cuprous  sulphide,  and 
weigh. 

Determination  of  Cadmium. — Evaporate  the  filtrate 
until  fumes  of  sulphuric  acid  appear.  Cool,  add  5  c.c.  of 
concentrated  hydrochloric  acid,  and  transfer  the  solution 
to  a  conical  flask,  using  hydrogen  sulphide  solution  to  rinse 
the  basin.  Then  add  slowly,  with  constant  stirring,  hydrogen 
sulphide  solution  until  the  volume  is  increased  to  about  150 
c.c.  Saturate  the  solution  with  hydrogen  sulphide,  collect 
the  cadmium  sulphide,  and  convert  it  into  sulphate  as 
described  on  p.  174. 

Determination  of  Iron. — Evaporate  the  filtrate  from  the 
cadmium  sulphide  until  the  volume  is  reduced  to  about  50 
c.c.,  transfer  to  a  beaker,  add  20  c.c.  of  concentrated  hydro- 
chloric acid,  and  dilute  to  100  c.c.  Determine  the  iron  in 
this  solution  by  the  "cupferron"  method  (p.  186).  (If 
preferred,  the  basic  acetate  method,  with  a  double  pre- 
cipitation, may  be  used  to  separate  the  iron  from  the  zinc 
and  manganese.  (Cf.  p.  165.) 

Determination  of  Manganese. — In  the  filtrate  from  the 
iron  precipitation,  determine  the  manganese  as  described 
on  p.  191. 

Determination  of  Zinc. — After  removal  of  manganese, 
determine  the  zinc  as  zinc  ammonium  phosphate.  For 
details,  see  p.  216. 


248       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Determination  of  Sulphur. — In  a  separate  portion  of  the 
zinc  blende,  determine  the  sulphur  as  described  on  p.  242, 
or  p.  244. 

Determination  of  Carbonate. — In  a  separate  portion 
(2  to  5  grams)  of  the  zinc  blende,  determine  the  amount  of 
carbonate.  As  the  blende,  on  treatment  with  acid,  loses 
hydrogen  sulphide  as  well  as  carbon  dioxide,  the  indirect 
method  is  not  applicable.  The  direct  method  (p.  175)  must 
be  used.  Two  U-tubes,  each  containing  a  concentrated 
solution  of  copper  sulphate,  must  be  placed  next  to  the 
reaction  flask  in  order  to  absorb  the  hydrogen  sulphide. 


ANALYSIS    OP   PYROLUSITB   OR   OP   MANQANITB. 

Pyrolusite  and  manganite  are  the  commonest  natural 
ores  of  manganese.  The  former  consists  mainly  of  man- 
ganese dioxide,  and  the  latter  mainly  of  the  hydrated  oxide, 
Mn2O3,H2O.  Traces  of  "gangue,"  ferric  oxide,  and  barium 
oxide  are  usually  present  in  both  minerals. 

OUTLINE  OF  METHOD. — One  portion  of  the  mineral  is  extracted  with 
hydrochloric  acid,  and  the  insoluble  residue  of  silica,  etc.,  separated. 
Barium  is  separated  as  barium  sulphate,  and  the  iron  is  then  pre- 
cipitated by  means  of  "  cupferron."  After  removal  of  the  iron,  the 
manganese  is  determined  as  manganese  dioxide. 

In  another  portion  of  the  mineral,  the  manganese  dioxide  is 
determined  by  a  volumetric  method. 

Water  is  determined  in  a  separate  portion  of  the  mineral. 

Determination  of  Barium,  Iron,  and  Manganese. 

To  a  weighed  portion  (about  0-5  gram)  of  the  finely 
powdered  mineral  in  a  conical  flask,  add  10  c.c.  of  water 
and  20  c.c.  of  concentrated  hydrochloric  acid,  and  close  the 
flask  loosely  with  a  funnel  or  glass  bulb  in  order  to  prevent 
loss  by  spirting.  Warm  on  the  steam-bath  until  the  residue 
is  white,  and  then  evaporate  to  complete  dryness  in  a 
porcelain  basin.  Drench  the  dry  residue  with  concentrated 
hydrochloric  acid,  set  aside  for  five  minutes,  and  then  dilute 
to  about  30  c.c.  Filter  through  a  small  filter  paper;  wash 
the  residue,  and  ignite  it.  Weigh,  and  report  the  result  as 
"  insoluble  residue." 


PYROLUSITE  249 

Determination  of  Barium. — Heat  the  filtrate  until  boil- 
ing, and  to  the  hot  solution  add  I  c.c.  of  dilute  sulphuric  acid. 
Filter  through  a  small  filter  paper,  wash  with  hot  water, 
incinerate  the  paper  with  the  precipitate,  and  weigh  the 
BaSO4.  The  quantity  of  barium  in  these  minerals  is,  as  a 
rule,  so  small  that  the  error  due  to  the  retention  of  ferric 
oxide  by  the  barium  sulphate  is  negligible. 

Determination  of  Iron. — Precipitate  the  iron  in  the 
filtrate  by  means  of  "cupferron,"  as  described  on  p.  186. 
The  precipitate  is  liable  to  contain  traces  of  manganese, 
but,  as  the  amount  of  iron  in  these  minerals  is  usually  very 
small,  the  error  is  negligible.  Filter,  and  wash  the  precipi- 
tate ;  ignite,  and  weigh  the  ferric  oxide. 

Determination  of  Manganese. — After  the  removal  of 
iron,  precipitate  the  manganese,  together  with  any  traces 
of  calcium,  etc.,  as  carbonate  (p.  190).  Filter,  and  wash 
thoroughly.  Perforate  the  filter  paper  with  a  glass  rod, 
and,  by  means  of  a  jet  of  water,  wash  the  precipitate  into 
a  beaker.  Cover  the  beaker  with  a  clock-glass,  and  dissolve 
the  crude  manganous  carbonate  in  dilute  sulphuric  acid ; 
wash  the  filter  paper  with  a  little  warm  dilute  sulphuric  acid 
in  order  to  dissolve  any  traces  of  precipitate  adhering  to  it, 
and  add  these  washings  to  the  main  solution.  Dilute  the 
solution  to  about  200  c.c.  and  precipitate  the  manganese  as 
dioxide  (p.  191). 

Determination  of  Manganese  Dioxide. — Determine  the 
amount  of  manganese  dioxide  by  the  method  described 
on  p.  92. 

If  the  amount  of  manganese  found  in  this  way  is  less  than 
the  total  manganese  as  determined  gravimetrically,  calculate 
the  excess  of  manganese,  above  that  present  as  the  dioxide, 
as  MnO. 

Determination  of  Water. — Determine  the  amount  of 
water  in  a  separate  portion  of  the  mineral  by  the  method 
described  on  p.  215. 

ANALYSIS  OP  SUPERPHOSPHATE  MANURE. 

Superphosphate  manure  is  prepared  from  natural  phos- 
phate, bone  dust,  or  basic  slag,  by  treatment  with  sulphuric 


250       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

acid.  By  this  treatment,  insoluble  tricalcium  phosphate, 
Ca3(PO4)2,  is  converted  into  soluble  acid  phosphate, 
CaH4(PO4)2.  The  value  of  a  superphosphate  as  a  plant 
fertiliser  depends  mainly  on  the  amount  of  soluble  phos- 
phate present.  It  is  therefore  often  sufficient  to  determine 
only  the  respective  amounts  of  "soluble"  and  "insoluble" 
phosphates.  The  following  scheme  for  a  more  complete 
examination  of  a  superphosphate  is  sufficiently  comprehen- 
sive for  most  purposes,  but  is  not  put  forward  as  providing 
a  complete  method  of  analysis.  On  account  of  the  many 
admixtures  to  be  found  in  superphosphates,  any  general 
scheme  of  analysis  would  be  very  unwieldy. 

OUTLINE  OF  METHOD. — Separate  portions  of  the  superphosphate  are 
used  for  the  determination  of: — (i)  "  Silicious  matter,"  iron, 
aluminium,  and  calcium;  (2)  Sulphate;  (3)  "Soluble"  and  "in- 
soluble "  phosphate  ;  (4)  Water,  organic  matter,  and  sodium  and 
potassium  ;  (5)  Total  nitrogen. 

The  sample  should  be  thoroughly  mixed  in  order  to 
secure  uniformity,  but  it  should  not  be  dried  or  ground. 
All  results  should  be  expressed  in  percentages  of  the 
original,  undried,  material. 

Determination  of  Iron,  Aluminium,  and  Calcium. 

Place  2  grams  (accurately  weighed)  of  the  superphosphate 
in  a  porcelain  basin,  add  20  c.c.  of  water  and  5  c.c.  of  con- 
centrated hydrochloric  acid,  and  evaporate  to  complete  dry- 
ness  (cf.  p.  229).  Moisten  the  residue  with  concentrated 
hydrochloric  acid.  Set  aside  for  five  minutes,  then  dilute  to 
about  30  c.c.,  and  filter.  Wash  the  residue  at  first  with  hot 
dilute  hydrochloric  acid  and  then  with  hot  water.  If  much 
calcium  sulphate  is  present,  prolonged  washing  with  dilute 
acid  is  necessary  to  dissolve  it.  Dry,  ignite,  and  weigh  the 
"insoluble  residue." 

Determination  of  Iron  and  Aluminium. — Add  sodium 
hydroxide  solution  to  the  filtrate  until  it  is  alkaline,  then  add 
a  few  crystals  of  potassium  nitrate,  and  evaporate  to  dryness 
in  a  porcelain  basin.  Ignite  the  residue  gently  in  order  to 
destroy  organic  matter.  Moisten  the  residue  with  concen- 
trated hydrochloric  acid,  add  30  c.c.  of  water,  and  warm. 
Transfer  the  solution  to  a  beaker  and  dilute  to  about  200  c.c. 


SUPERPHOSPHATE  MANURE  251 

Heat  the  solution  until  almost  boiling,  and  add  ammonia 
until  it  is  slightly  alkaline.  The  precipitate  obtained  con- 
sists mainly  of  ferric,  aluminium,  and  calcium  phosphates. 
Add  i  c.c.  of  phenolphthalein  solution  and  then  acetic  acid 
until  the  colour  is  completely  discharged.  Filter,  and  wash 
with  hot  water.  Dissolve  the  precipitate  in  hot,  concentrated 
hydrochloric  acid,  dilute  the  solution  to  about  50  c.c.,  and 
warm  it.  To  the  warm  solution,  add  ammonia  until  alkaline, 
and  then  acidulate  with  acetic  acid.  Filter,  and  wash  with 
hot  water.  Incinerate  the  paper  apart  from  the  precipitate, 
add  the  precipitate,  and  ignite  gently.  Weigh  the  mixture 
of  A1PO4  and  FePO4.  After  weighing,  dissolve  the  mixed 
phosphates  in  sulphuric  acid,  and  determine  the  iron 
volumetrically.  For  most  purposes,  it  is  sufficiently  accurate 
to  assume  that  the  precipitate  is  a  pure  mixture  of  FePO4 
and  A1PO4.  On  this  assumption,  the  amount  of  aluminium 
can  be  calculated  from  the  weight  of  the  mixed  phosphate 
and  the  amount  of  iron  in  it. 

Determination  of  Calcium. — Mix  the  filtrates  from  the 
two  precipitations,  and  determine  the  calcium  by  precipita- 
tion as  oxalate  (p.  230).  The  filtrate  from  the  calcium 
oxalate  contains  the  magnesium,  and,  if  desired,  it  may  be 
determined  as  described  on  p.  231. 

Determination  of  Sulphate. 

Extract  a  weighed  portion  (about  i  gram)  of  the  super- 
phosphate with  hydrochloric  acid,  evaporate  to  dryness,  and 
prepare  a  solution  as  described  above.  In  the  filtrate  from 
the  insoluble  residue,  determine  the  sulphate  as  described 
on  p.  131. 

Determination  of  "Soluble"  and   "Insoluble"  Phosphate. 

Place  10  grams  of  the  superphosphate  in  a  500  c.c.  flask, 
and  add  400  c.c.  of  water.  Shake  vigorously  by  means  of  a 
shaking  machine  for  thirty  minutes  (150  revolutions  per 
minute  is  recommended  as  the  standard  speed  for  the 
shaking  apparatus).  Dilute  the  solution  to  500  c.c.  and 
filter  immediately.  Drain  the  residue  as  much  as  possible, 
but  do  not  wash  it.  Examine  the  residue  and  solution 
separately. 


252       ANALYSIS  OF  SIMPLE  ORES  AND  ALLOYS 

Determination  of  "Soluble  "  Phosphate. — Determine  the 
phosphate  in  25  c.c.  of  the  solution  (corresponding  to  0-5 
gram  of  the  superphosphate)  by  the  molybdate  method 
(p.  197).  It  is  conventional  to  calculate  the  phosphate 

as  Ca3(P04)2- 

Determination     of    "Insoluble"    Phosphate. — To     the 

insoluble  part  add  about  80  c.c.  of  water  and  20  c.c.  of 
concentrated  nitric  acid,  and  evaporate  to  complete  dryness. 
Cover  the  dry  residue  with  concentrated  nitric  acid,  and  set 
aside  for  five  minutes  before  diluting  and  filtering.  Wash 
with  hot  dilute  nitric  acid  and  then  with  hot  water.  Dilute 
the  filtrate  to  500  c.c.,  and  determine  the  phosphate  in 
25  c.c.  of  this  solution  by  the  molybdate  method. 

Determination  of  Water,  Organic  Matter,  Sodium, 
and  Potassium. 

Determination  of  Water. — Spread  in  a  thin  layer  a 
weighed  portion  (2  to  5  grams)  of  the  superphosphate  in  a 
platinum  basin,  and  dry  for  five  hours  at  100°.  Report  the 
loss  of  weight  as  "  moisture." 

Place  the  dried  sample  in  an  air-oven,  and  dry  at  160°  to 
170°  until  the  weight  is  constant  Report  the  further  loss  of 
weight  as  "  combined  water." 

Determination  of  Organic  Matter. — To  the  dried  residue 
add  saturated  barium  hydroxide  solution,  mixing  thoroughly 
with  a  glass  rod,  until  the  solution  is  alkaline.  Evaporate 
to  dryness  on  the  steam-bath,  dry  at  160°  to  170°  until  the 
weight  is  constant.  Then  heat  on  a  sand-bath,  gently  at 
first,  and  finally  for  fifteen  minutes  to  barely  visible  redness. 
Cool  in  a  desiccator,  and  weigh.  The  loss  of  weight  resulting 
from  the  ignition  is  the  weight  of  the  organic  matter. 

Determination  of  Sodium  and  Potassium. — If  sodium 
and  potassium  are  to  be  determined,  it  is  convenient  to 
use  the  residue  after  the  above  treatment.  Proceed  in  the 
same  manner  as  with  the  ignited  mass  obtained  in  the 
determination  of  the  alkalis  by  the  Lawrence  Smith 
method  (p.  235). 

Determination  of  Nitrogen. 

Determine  the  nitrogen  in  a  weighed  portion  (2  to  5  grams) 
of  the  superphosphate  by  Kjeldahl's  method  (p.  334). 


PART   VII 
GAS   ANALYSIS 

THE  analysis  of  a  gas  is  usually  conducted  in  one  or  other 
of  two  ways. 

(1)  A  measured   volume   of  the  gas  is   treated    with   a 
suitable    absorbing-reagent    and     the     change    of    volume 
noted,  or 

(2)  A  measured  volume  of  the   gas   is   treated    with   a 
suitable  reagent,  and  the  constituent  thus  absorbed  is  then 
determined  in  the  reagent.     This    is  the  usual  method  for 
the    determination    of    traces    of    one    constituent,    since 
large  volumes  of  gas  may  be  used ;   it  is  also   the   general 
method  for  gases  which  are  readily  soluble   in  water,  such 
as  sulphur  dioxide. 

In  all  cases,  accurate  measurement  of  gas  volumes  forms 
part  of  the  process,  and  a  knowledge  of  the  laws  regulating 
the  volume  of  a  gas  under  varying  conditions  is  therefore 
necessary.  Gases  are,  in  practice,  always  measured  in 
contact  with  mercury  or  water.  The  vapour  pressure  of 
mercury  is  so  small  at  ordinary  temperatures  that  it  may 
be  neglected  except  in  the  most  exact  work.  The  vapour 
pressure  of  water  is  much  greater,  and  cannot  be  neglected 
in  accurate  work. 

The  volume  of  a  gas  is  determined  as  a  rule  in  the  moist 
state  at  the  room  temperature  and  at  barometric  pressure. 
Corrections  are  then  applied  to  find  the  volume  the  gas 
would  occupy  in  the  dry  state  at  o°  and  760  mm.  The 
correction  for  the  volume  occupied  by  the  water  vapour  is 
applied  by  subtracting  the  vapour  pressure  of  water  (at  the 
experimental  temperature)  from  the  observed  barometric 
pressure.  If  v  is  the  observed  volume  of  the  gas,  t  its 

253 


254  GAS  ANALYSIS 

temperature,  /  the  barometric  pressure  in  mm.,  and  w  the 
vapour  pressure  of  water  at  temperature  /,  the  corrected 
volume,  V,  is  found  from  the  following  equation  :  — 


v  =  v 

760(273  +  0 


or  V  =  v 


760(1+0-00367  f) 


Technical  Methods. — Analyses  can  be  performed  rapidly 
and  with  sufficient  accuracy  for  most  purposes  by  using 
apparatus  (designed  mainly  by  Hempel)  with  water  as  the 
confining  liq-uid.  In  these  analyses,  the  above-mentioned 
corrections  are  neglected,  since  the  error  introduced  in  this 
way  is  not  larger  than  other  errors  inherent  in  the  same 
methods.  It  is  assumed,  in  fact,  that  the  temperature  and 
pressure  remain  constant  during  the  analysis.  It  is  unlikely 
that  the  barometric  pressure  will  alter  sufficiently  during 
an  analysis  to  introduce  any  serious  error,  but  care  is 
necessary  if  the  temperature  variation  is  to  be  kept  within 
sufficiently  narrow  limits.  Obviously,  the  temperature  will 
alter  if  any  of  the  apparatus  is  brought  near  a  flame,  radiator, 
or  other  source  of  heat,  or  is  exposed  to  direct  sunlight. 
For  the  same  reason,  the  apparatus  must  be  lifted  by  the 
support,  and  the  glass  parts  of  the  apparatus  must  not  be 
touched  more  than  is  necessary  with  the  hands. 

For  more  exact  work,  with  mercury  as  the  confining 
liquid,  see  Hempel's  Methods  of  Gas  Analysis,  translated  by 
L.  M.  Dennis  (Macmillan  &  Co.). 

COLLECTION  OP  A  SAMPLE  OP  GAS   FOR   ANALYSIS. 

If  a  large  quantity  of  the  gas  is  available,  it  is  most 
convenient  to  fill  a  tube  or  other  vessel  by  displacement  of 
the  air  originally  present,  care  being  taken  that  the  air  is 
completely  displaced. 

When  the  quantity  of  gas  is  limited,  the  receiver  must 
be  filled  with  water  or  mercury,  which  is  then  displaced  by 
the  gas.  An  inverted  wash-bottle  with  a  piece  of  rubber 
tubing  and  a  screw-clip  on  each  of  the  tubes  may  be  used 


COLLECTION  OF  A  SAMPLE 


255 


FIG.  57. 


for  the  collection  of  a  gas  (Fig.  57).  In  the  laboratory, 
it  is  usually  possible  to  collect  the  gas  directly  in  the 
gas-burette  (see  below). 

Samples  of  air  in  mines,  etc.,  are  conveniently  collected 
in  small  (100  to  200  c.c.)  glass-stoppered 
bottles.  To  collect  the  sample,  remove 
the  glass  stopper,  insert  a  rubber  cork 
fitted  with  glass  inlet  and  outlet  tubes, 
and  blow  air  through  the  bottle  by 
means  of  a  small  bellows  or  other 
simple  air-pump.  If  an  air-pump  is 
not  available,  a  sample  may  be  taken 
by  drawing  air  through  the  bottle  by 
mouth-suction ;  there  are,  however, 
obvious  objections  to  this  method. 
Whatever  method  is  used,  the  operator 
should  keep  as  far  as  possible  from  the 
inlet  tube  while  the  sample  is  being  taken,  in  order  to 
minimise  the  risk  of  contaminating  the  sample  with  expired 
air. 

Withdraw  the  rubber  cork,  and  immediately  insert  the 
glass  stopper  which  is  lubricated  with  sufficient  vaseline  to 
render  the  bottle  air-tight.  The  stopper  should  be  kept  in 
position  by  means  of  a  stout  rubber  band.  The  transference 
of  the  gas  from  the  bottle  to  a  gas-burette  is  described 
on  p.  259. 

In  connection  with  both  the  collection  and  the  analysis 
of  a  gas,  two  points  require  special  mention,  firstly,  the 
solubility  of  gases  in  water  and  aqueous  solutions,  and 
secondly,  the  permeability  of  rubber  to  gases.  All  gases 
are  soluble  in  water  and  in  aqueous  solutions,  but  to  very 
different  extents.  The  solubility  of  nitrogen  and  oxygen 
is  too  small  to  affect  an  analysis  seriously,  particularly  as  the 
reagents  are  already  saturated  with  these  gases  at  their 
respective  atmospheric  partial  pressures. 

With  most  other  gases,  the  reagents  should  be  saturated 
with  each  gas  present  in  the  mixture  at  the  pressure  corre- 
sponding to  its  partial  pressure.  This  is  accomplished  with 
sufficient  exactness  for  most  purposes  by  carrying  out 
several  successive  analyses  of  the  same  sample ;  the  error 


256  GAS  ANALYSIS 

from  this  source  will  then  diminish  with  each  successive 
analysis,  and  in  most  cases  the  second  or  third  analysis  will 
be  sufficiently  accurate.  An  exceptional  case,  however,  is 
carbon  dioxide,  which  is  so  soluble  that  it  cannot  be 
accurately  determined  with  any  apparatus  in  which  water 
is  the  confining  liquid. 

From  the  impermeability  of  rubber  to  water,  one  might 
presume  that  it  would  also  be  impermeable  to  gases,  but 
this  is  by  no  means  the  case.  All  gases  will  pass  through 
a  rubber  membrane  even  if  it  is  free  from  flaws.  The  rate 
of  diffusion  through  rubber  varies  for  different  gases,  and 
is  fast  enough  in  the  case,  for  example,  of  carbon  dioxide 
to  introduce  a  serious  error  if  the  gas  is  exposed  for  any 
considerable  time  to  a  rubber  wall.  In  practice,  therefore, 
all  rubber  connections  are  kept  as  short  as  possible.  Old 
or  parched  rubber  should  never  be  used,  and  all  rubber  tubes 
should  be  tested  for  leaks  from  time  to  time.  Pinchcocks 
and  clips  of  any  kind  should  be  removed  when  the 
apparatus  is  not  in  use,  as  the  rubber  is  thereby  kept  in 
better  condition. 


Gas  Analysis  with  Hempel   Apparatus 

The  sample  of  gas  to  be  analysed  is  introduced  into 
a  gas-burette,  in  which  it  is  measured.  It  is  then  led  into 
a  gas-pipette,  in  which  it  is  treated  with  a  reagent  which 
absorbs  one  constituent  of  the  mixture.  The  residue  is 
brought  back  into  the  gas-burette  and  the  volume  again 
measured,  the  contraction  giving  the  volume  of  the  con- 
stituent absorbed.  The  residue  is  then  led  into  another 
gas-pipette,  where  it  is  treated  with  another  reagent  which 
absorbs  a  second  constituent,  and  the  residual  gas  is  again 
brought  back  to  the  burette  for  measurement.  This  series 
of  operations  is  continued,  using  as  many  pipettes  as  there 
are  constituents  to  be  determined. 

THE   GAS-BURETTE. 

This  consists  of  a  graduated  measuring-tube  M,  and  an  un- 
graduated  levelling-tube  L  (Fig.  58).  Each  tube  is  supported 
in  an  upright  position  by  a  stand,  and  the  lower  ends 
are  connected  by  a  rubber  tube  K  of  sufficient  length  to 
allow  one  tube  to  be  placed  on  the  floor  while  the  other 
is  on  the  working  bench.  The  measuring-tube  is  graduated  in 
fifths  of  a  cubic  centimetre,  from  o  to  100  c.c.  It  terminates  at 
the  upper  end  in  a  short  capillary  tube  to  which  a  rubber  tube 
R  is  attached.  This  rubber  tube  should  be  securely  wired 
on  to  the  glass  capillary  tube,  leaving  about  3  cm.  of  the 
rubber  tube  projecting. 

To  prepare  the  burette  for  an  analysis,  pour  into  the 
levelling-tube  sufficient  water  to  fill  one  of  the  glass  tubes 
and  the  connecting  rubber  tube.  In  order  to  make  sure 
that  there  is  no  air  in  the  rubber  tube,  run  the  water  to  and 
fro  in  the  tubes  by  alternately  raising  and  lowering  one  of 
them.  Then  raise  the  levelling-tube  until  the  measuring- 
tube  and  rubber  tube  R  are  completely  filled  with  water, 

257  R 


258 


GAS  ANALYSIS 


and  close  the  rubber   tube  with  a  clip,  placed  as  near  the 
glass  as  possible. 

Introduction  of  a  Sample  into  the  Burette. — Fill  the 
measuring-tube  with  water  as  described  above,  and  close 
the  clip  on  the  tube  R.  If  the  rubber  tube  R  is  not  quite 
full,  fill  it  from  a  wash-bottle.  Insert 
into  the  rubber  tube  a  well-fitting, 
capillary-bored,  glass  tube  leading  from 
the  vessel  containing  the  gas  to  be 
analysed.  All  air  in  the  leading-tube 
must  have  been  previously  expelled  by 
passing  some  of  the  gas  through  it  or 
by  filling  it  with  water.  Open  the  clip 
on  the  tube  R  and,  by  lowering  the 
levelling-tube,  allow  the  gas  to  enter 
until  the  burette  contains  a  little  more 
than  loo  c.c.  Close  the  clip  on  R,  and 
disconnect  the  leading-tube. 

It  is  convenient  to  work  with  exactly 
100   c.c.    of    gas,    and    this    is   readily 
measured  off  in  the  following  manner. 
Wait  for  two  minutes  until   the  water 
has    drained     from    the    side    of    the 
measuring-tube,  then  raise  the  levelling- 
tube  until   the    gas   is   compressed    to 
slightly  less  than  100  c.c.,  and  close  the 
rubber  connecting  tube 
K  by  pinching  between 
the  fingers.    Lower  the 
levelling-tube  and,  by 
cautiously  relaxing  the 
pressure  of  the  fingers 


R 

<^^ 

I 

o 

L 

M 

FIG.  58. 


on  the  tube  K,  allow  the  gas  to  expand  until  the  volume 
is  exactly  100  c.c. ;  then  pinch  the  rubber  tube  tightly  again 
and  open  the  clip  on  R  for  a  moment.  The  excess  of  gas 
thereby  escapes,  leaving  exactly  100  c.c.  at  atmospheric 
pressure.  Make  sure  that  the  volume  is  exactly  100  c.c.,  by 
equalising  the  levels  of  the  water  in  the  two  tubes  of  the 
burette  and  reading  the  volume  of  gas.  When  adjusting 
the  water  levels,  hold  the  levelling-tube  in  a  sloping  position 


HEMPEL  APPARATUS 


259 


and  bring  it  into  contact  with  the  measuring-tube.  The 
measuring-tube  must  be  vertical. 

The  operation  of  introducing  exactly  100  c.c.  of  gas  into 
the  burette  should  be  practised  with  air. 

Introduction  of  a  Sample  from  a  Small  Bottle. — Hold 
the  mouth  of  the  bottle  below  the  surface  of  some  mercury 
which  is  contained  in  a  deep  trough.  Remove  the  stopper, 


FIG.  59. 

care  being  taken  that  the  mouth  of  the  bottle  is  kept  well 
below  the  surface  of  the  mercury.  Fill  a  bent,  thick-walled, 
capillary  tube  with  water,  insert  it  into  the  bottle,  and  con- 
nect the  other  end  of  the  capillary  tube  with  the  gas-burette 
(Fig.  59).  The  burette  may  then  be  rilled  as  described  above. 

ABSORPTION   PIPETTES. 

Simple  Absorption  Pipette  for  Liquids. — This  consists 
of  two  glass  bulbs,  A  and  B,  con- 
nected by  a  wide  tube.  The  bulb  A 
is  of  about  150  c.c.  capacity  and  is 
connected  with  a  long  capillary  tube 
C,  which  is  bent  as  shown  in  Fig.  60. 
The  bulb  B  must  not  be  less  than 
1 20  c.c.  The  pipette  is  supported  on 
a  suitable  metal  or  wooden  stand.  A 
short  piece  of  good  rubber  tubing  is 
wired  on  to  the  open  end  of  the 
capillary  tube.  When  the  pipette  is 
not  in  use,  this  rubber  tube  should  be 
closed  with  a  glass  plug,  and  the  tube  D  should  also  be  closed 
with  a  small  rubber  stopper. 


D 


FIG.  60. 


260 


GAS  ANALYSIS 


1) 


The  pipette  is  filled  by  running  in,  through  the  tube  D, 
sufficient  of  the  reagent  to  fill  the  bulb  A  completely  and  the 
bulb  B  to  a  depth  of  about  I  cm. 

Simple  Absorption  Pipette  for  Solids  or  Liquids. — This 
differs  from  the  above  form  in  having 
a  tube  at  E  (Fig.  61)  through  which 
solid  reagents  may  be  introduced 
into  the  absorption  bulb  A.  The  tube 
E  is  closed  with  a  rubber  stopper 
which  should  be  securely  wired  in 
place.  With  this  pipette,  absorption 
with  liquid  reagents  can  be  greatly 
facilitated  by  packing  the  bulb  A 
with  rolls  of  wire  gauze  or  with  fine 
glass  rods  before  filling  the  pipette 
with  the  reagent.  When  the  gas  is 
then  introduced  into  A,  it  is  exposed  to  a  large  surface  of 
the  reagent. 

Double  Absorption  Pipettes. — Reagents,  such  as  cuprous 
chloride,  alkaline  pyrogallate,  etc.,  which  absorb  oxygen,  must 
not  be  used  in  the  simple  pipettes  described  above.  With 
these  reagents,  a  so-called  "double  pipette"  (Figs.  62  and  63) 


FIG.  61. 


H 


D 


FIG.  62. 


FIG.  63. 


must  be  used.  In  this  form  of  apparatus,  the  reagent  in  B 
comes  inrto  contact  with  an  atmosphere  free  from  oxygen, 
the  indifferent  gas  being  kept  in  place  by  water  in  the  bulbs 
F  and  G. 

The   filling   of  a    Hempel   double  pipette,  as  ordinarily 


REAGENTS  FOR  ABSORPTION  PIPETTES          261 

constructed,  offers  some  little  difficulty.  By  the  addition  of 
a  side-tube,  shown  in  the  diagram  at  H,  the  pipette  is  as 
easily  filled  as  a  simple  pipette.  The  pipette  is  first  filled 
with  an  indifferent  gas,  such  as  nitrogen  or  hydrogen,  by 
passing  it  through  while  the  tube  H  is  closed  with  a  cork. 
The  reagent  is  introduced  by  pouring  it  in  through  the 
tube  H,  and  water  is  poured  in  through  D  until  the  bulb  F 
is  almost  full — in  order  to  form  a  water-seal.  The  tube  H 
is  then  closed  with  a  well-fitting  rubber  cork. 

REAGENTS    USED    IN   ABSORPTION    PIPETTES. 

Potassium  Hydroxide.  (For  carbon  dioxide]. — Dissolve 
500  grams  (or  the  contents  of  a  i  Ib.  bottle)  in  I  litre  of 
water,  and  store  in  a  bottle  with  a  rubber  stopper.  Pack 
the  absorption  bulb  of  a  simple  pipette  (Fig.  61)  with  rolls 
of  iron  wire  gauze  of  wide  mesh.  Fill  with  the  above 
solution  of  potassium  hydroxide.  All  the  carbon  dioxide  is 
absorbed  within  two  or  three  minutes.  When  wet  with  this 
solution,  the  iron  does  not  take  up  any  oxygen  by  oxidation. 

Refill  the  pipette  after  about  500  c.c.  of  carbon  dioxide 
has  been  absorbed. 

Bromine.  (For  ethylene  and  other  unsaturated  hydro- 
carbons^ and  benzene). — Fill  a  double  pipette  (Fig.  62)  with 
a  saturated  aqueous  solution  of  bromine,  and  pour  in  a  few 
cubic  centimetres  of  liquid  bromine  to  ensure  that  the 
solution  remains  saturated. 

After  the  gas  has  been  in  contact  with  the  reagent  in  the 
pipette  for  five  minutes,  draw  it  back  into  the  burette,  and 
then  pass  it  into  a  potassium  hydroxide  pipette  in  order  to 
remove  bromine  vapour ;  finally,  return  the  gas  to  the  burette, 
and  measure  it. 

Fuming  Sulphuric  Acid.  (For  ethylene  and  other  un- 
saturated hydrocarbons^  and  benzene). — This  reagent  absorbs 
the  same  gases  as  bromine.  A  simple  pipette  filled  with 
glass  beads  or  rods  may  be  used,  but  the  special  form,  with 
three  bulbs,  shown  in  Fig.  64,  is  recommended.  The 
pipette  is  filled  as  usual,  but  great  care  must  be  taken  that 
the  reagent  does  not  come  into  contact  with  the  rubber 
connections  or  with  water.  The  connecting-tubes  must  not 
be  filled  with  water,  and,  when  the  gas  is  transferred  to  the 


262  GAS  ANALYSIS 

pipette,  care  must  be  taken  to  prevent  any  water  passing 
over  into  the  pipette.  The  absorption  is  complete  within  five 
minutes.  When  the  gas  is  withdrawn 
for  measurement,  allow  the  fuming 
sulphuric  acid  to  follow  until  it  reaches 
a  mark  made  on  the  pipette  capillary, 
showing  the  position  of  the  reagent 
at  the  start  of  the  experiment.  It 
is  evident  that  an  error  is  introduced 
on  account  of  the  air  in  the  capil- 
lary. If  the  volume  is  known,  a 

correction    may   be   applied,  but  for 
FIG.  64.  '  ,  .          ..  .,, 

many  purposes  the  error  is  negligible; 

in  any  case  the  error  affects  only  the  oxygen  and  nitrogen 
determinations  to  any  appreciable  extent.  During  the  ab- 
sorption, sulphur  dioxide  is  formed,  and  fuming  sulphuric 
acid  itself  has  a  high  vapour  pressure ;  these  vapours  must 
be  removed  by  passing  the  gas  into  a  potassium  hydroxide 
pipette  before  measurement. 

The  pipette  must  be  refilled  after  about  i  litre  of  ethylene 
has  been  absorbed. 

Alkaline  Pyrogallate.  (For  oxygen}.  — Dissolve  separately 
7  grams  of  pyrogallol  in  25  c.c.  of  water,  and  50  grams  of 
potassium  hydroxide  in  no  c.c.  of  water.  Mix  the  two 
solutions  and  introduce  at  once  into  a  double  pipette. 
Potassium  hydroxide  purified  by  alcohol  must  not  be  used. 

The  reagent  absorbs  oxygen  very  slowly  at  temperatures 
below  10°,  but  the  absorption  is  rapid  and  complete  at 
higher  temperatures.  The  pipette  must  be  refilled  after 
about  200  c.c.  of  oxygen  has  been  absorbed.  The  reagent 
will  absorb  oxygen  in  presence  of  ethylene,  ammonia,  and 
other  substances  which  interfere  with  the  absorption  of 
oxygen  by  phosphorus. 

Phosphorus.  (For  oxygen). — Ordinary  yellow  phosphorus 
is  cast  in  thin  sticks l  with  which  the  bulb  of  a  simple  pipette 

1  To  obtain  fine  sticks  of  phosphorus,  melt  it  under  water  at  about 
50°  in  a  narrow  beaker,  using  enough  phosphorus  to  fill  the  beaker  to  a 
depth  of  6  or  7  cm.  Dip  a  narrow  glass  tube  into  the  molten  phosphorus, 
close  the  top  of  the  tube  with  the  finger,  and  plunge  the  tube  into  cold 
water.  The  solidified  phosphorus  is  readily  removed. 


REAGENTS  FOR  ABSORPTION  PIPETTES          263 

(Fig.  61)  is  packed.  The  pipette  is  filled  with  water,  and  the 
bulb  is  covered  with  a  metal  cover  to  protect  the  phosphorus 
from  the  action  of  light. 

Absorption  of  oxygen  is  marked  by  the  appearance  of 
white  clouds  of  the  oxide,  and  is  complete  within  five  minutes 
if  the  temperature  is  above  15°;  at  lower  temperatures  the 
absorption  is  much  slower,  and,  in  cold  weather,  absorption 
sometimes  does  not  occur  at  all  unless  the  water  in  the 
pipette  is  warmed. 

No  absorption  of  oxygen  takes  place  if  the  partial  pressure 
of  the  oxygen  is  greater  than  about  0-5  atmosphere,  or  if  the 
mixture  contains  ethylene,  heavy  hydrocarbons,  ammonia,  or 
alcohol,  even  in  traces.  A  mixture  containing  above  50  per 
cent,  of  oxygen  may  be  diluted  with  a  known  volume  of 
nitrogen,  and  the  oxygen  will  then  be  absorbed. 

Phosphorus  is  much  cleaner  to  work  with  than  alkaline 
pyrogallate,  and  the  pipette  can  be  used  for  scores  of  analyses. 
The  water  must  be  changed  from  time  to  time. 

Sodium Hydrosulphite.  (For oxygen). — Dissolve  25  grams 
of  sodium  hydrosulphite  in  120  c.c.  of  water  and  add  30  c.c. 
of  50  per  cent,  sodium  hydroxide.  Introduce  the  solution  at 
once  into  a  double  pipette.  The  reaction  which  occurs  in  the 
absorption  of  oxygen  is  represented  by  the  equation — 

Na,S204  +  H20  +  02  =  NaHS03  +  NaHSO4. 

The  absorption  is  complete  within  five  minutes  even  at  low 
temperatures,  and  is  unaffected  by  ethylene,  ammonia,  etc. 
The  pipette  should  be  refilled  after  about  300  c.c.  of  oxygen 
has  been  absorbed. 

Ammoniacal  Cuprous  Chloride.  (For  carbon  monoxide). — 
Dissolve  15  grams  of  cuprous  chloride  and  10  grams  of 
ammonium  chloride  in  the  minimum  quantity  of  con- 
centrated ammonia  solution,  and  dilute  to  200  c.c.  Transfer 
the  solution  at  once  to  a  double  pipette,  filled  with  rolls  of 
copper  gauze. 

This  reagent  is  used  for  the  absorption  of  carbon  monoxide, 
but,  as  it  also  absorbs  carbon  dioxide,  oxygen,  and  ethylene, 
these  gases  must  have  been  previously  removed.  Carbon 
monoxide  forms  with  the  reagent  a  compound  which  has  an 
appreciable  dissociation  pressure,  and  it  is  therefore  advisable 


264  GAS  ANALYSIS 

to  pass  the  gas  through  two  pipettes ;  the  first  may  contain 
reagent  which  has  been  used  several  times,  but  the  reagent 
in  the  second  pipette  should  be  as  fresh  as  possible.  Before 
measuring,  pass  the  gas  into  a  pipette  filled  with  dilute 
sulphuric  acid  in  order  to  remove  the  ammonia  which  escapes 
from  the  reagent.  Then  repeat  the  process,  omitting  the  first 
cuprous  chloride  pipette,  until  the  volume  is  constant. 

MANIPULATION   OP    APPARATUS. 

The  sample  of  gas  is  introduced  into  the  burette  as 
already  described,  and  the  burette  is  then  connected  with 
an  absorption  pipette  by  means  of  a  narrow  capillary  tube 
N,  as  shown  in  Fig.  65.  The  manipulation  is  as  follows : — 

Fill  the  open  end  of  the  rubber  tube  R  with  water  and 
insert  the  capillary  tube  N,  bringing  it  as  close  to  the  clip  as 
possible.  During  this  operation  the  capillary  tube  N  should 
become  filled  with  water ;  if  it  does  not,  repeat  the  operation 
after  putting  more  water  in  the  tube  R. 

Attach  a  piece  of  rubber  tubing  to  the  tube  D,  and  blow 
gently  until  the  liquid  has  filled  the  capillary  tube  C.  Place 
a  piece  of  fairly  stout  metal  wire  loosely  in  the  rubber  tube 
S,  insert  the  capillary  tube  N,  and  push  it  down  until  the 
end  is  in  contact  with  the  end  of  the  pipette.  Withdraw 
the  wire,  the  purpose  of  which  is  to  allow  air  to  escape  while 
the  capillary  tube  is  being  pushed  into  place.  The  connecting 
tube  N  and  the  capillary  tube  of  the  pipette  should  be  almost, 
if  not  entirely,  free  from  enclosed  air.  An  air  column  in  the 
capillary  may  be  neglected  if  not  above  I  cm.  long ;  if  more 
air  than  this  is  enclosed  the  operation  must  be  repeated 
more  carefully. 

The  clip  closing  the  tube  R  is  then  opened  and  the  gas  is 
driven  into  the  absorption  pipette  by  raising  the  levelling- 
tube  L.  Water  is  allowed  to  flow  over  from  the  burette  until 
it  fills  the  connecting  tube  and  the  capillary  tube  of  the 
pipette ;  when  the  water  reaches  the  end  of  the  capillary  at 
the  top  of  the  bulb  A,  the  clip  on  R  is  closed.  The  measuring- 
tube  M  is  fastened  to  the  pipette  stand  by  the  clamp  P, 
and  the  gas  is  then  brought  into  intimate  contact  with  the 
reagent  by  rocking  the  whole  apparatus  gently  backwards 
and  forwards.  (Do  not  shake  up  and  down.) 


HEMPEL  APPARATUS 


265 


After  shaking  for  five  minutes,  lower  the  levelling-tube, 
open  the  clip  on  R,  and  bring  the  gas  back  into  the  burette, 
allowing  the  reagent  to  flow  after  it  until  the  capillary  and 
connecting  tubes  are  filled  with  liquid.  None  of  the  reagent 
must  be  allowed  to  pass  into  the  burette.  Allow  two  minutes 


D 


FIG.  65. 

for  the  liquid  to  drain  from  the  sides  of  the  burette.  (A 
small  sand-glass  is  convenient  for  measuring  the  necessary 
time.)  Equalise  the  levels  of  the  water  in  the  two  tubes  and 
read  the  new  volume  of  the  gas.  Some  gases  are  rapidly 
absorbed  by  the  reagent  in  the  pipette,  whilst  others  require 
a  longer  time  ;  and,  to  ensure  that  the  absorption  is  complete, 
the  operation  should  therefore  be  repeated,  several  times  if 


266  GAS  ANALYSIS 

necessary,  until  the  volume  of  the  gas  remains  unaltered  by 
treatment  with  the  reagent  in  the  pipette. 

If  the  pipette  contains  a  solid  or  is  filled  with  wire  gauze 
or  glass  rods,  no  shaking  is  necessary,  on  account  of  the  much 
larger  surface  exposed  to  the  gas. 

The  difference  between  the  readings  before  and  after 
absorption  is  the  volume  of  gas  absorbed ;  and,  if  100  c.c.  was 
the  original  volume  of  the  gas,  the  difference  in  cubic  centi- 
metres is  the  percentage  of  the  constituent  in  the  mixture. 

ANALYSIS   OP   A   GASEOUS   MIXTURE. 

In  the  analysis  of  a  gaseous  mixture,  the  gases  must  be 
determined  in  a  definite  order.  The  order  in  which  the 
reagents  must  be  used  is  given  below  for  a  mixture  of  all 
the  common  gases ;  if  a  gas  is  known  to  be  absent,  the 
corresponding  reagent  will  of  course  be  omitted. 

I.  Potassium  hydroxide  .  .     for  carbon  dioxide. 

II.  (a)  Bromine;   or  (b)  Fuming  "j 
sulphuric  acid  (followed  in      r 
either    case    by    potassium  I 
hydroxide)    .  .  .  J 

III.  (a)  Sodium  hydrosulphite ;   or  1 

(b)  Phosphorus  ;  or  (c)  Alka-  I  for  oxygen. 
line  pyrogallate         .  .  J 

IV.  Ammoniacal  cuprous  chloride  ^ 

(followed  by  dilute  sulphuric  \  for  carbon  monoxide. 
acid)  .  .  J 

V.  The    unabsorbed    residue     is  \  for  methane,  hydro- 
analysed  as  described  below  J       gen,  and  nitrogen. 

Analysis  of  a  Mixture  of  Methane, 
Hydrogen,  and  Nitrogen. 

OUTLINE  OF  METHOD. — After  removal  of  all  other  gases,  the  methane 
and  hydrogen  are  burned  in  a  measured  volume  of  oxygen.  The 
products  of  combustion  are  removed,  the  contraction  in  volume  is 
noted,  and  the  amount  of  unused  oxygen  is  determined.  From  the 
data  obtained,  the  amounts  of  methane  and  hydrogen  can  be  cal- 
culated. The  residue  is  nitrogen. 

Procedure. — Other  gases  are  removed  in  the  usual 
manner.  The  residue  of  methane,  hydrogen,  and  nitrogen 


USE  OF  COMBUSTION  PIPETTE 


267 


is  transferred  to  a  slow-combustion  pipette  (Fig.  66).      In 

this   pipette,   the    methane   and   hydrogen    are   burned    by 

admitting  a  slow  stream  of  oxygen,  the  necessary  heat  being 

supplied      by     an      electrically 

heated    platinum    spiral.      The 

current    supplied  to   the   spiral 

should  be  just  sufficient  to  heat 

it   to  visible  redness — a   4-volt 

accumulator   serves   excellently 

with  the  spiral  usually  supplied 

with   the   pipette.      The   spiral 

should  be  about  I  cm.  from  the 

top  of  the  bulb. 

Pass  the  gas  into  the  pipette 
and  see  that  the  capillary  is 
completely  filled  with  water. 
Close  the  rubber  tube  on  the 
pipette  with  a  screw-clip,  dis- 
connect from  the  burette,  and  connect  with  a  second  burette 
containing  exactly  100  c.c.  of  oxygen.1 

At  this  stage  the  current  should  be  started.  After 
starting  the  heating  current,  raise  the  levelling-tube,  place  it 
on  a  tall  stand,  and  open  the  screw-clip  cautiously  so  that 
oxygen  is  driven  very  slowly  into  the  pipette.  The  oxygen 
should  pass  in  at  the  rate  of  about  5  c.c.  per  minute.  If  the 
oxygen  is  led  in  too  fast  an  explosion  will  occur.  While 
combustion  is  in  progress,  the  wire  will  glow  more  brightly  and 
the  volume  of  gas  in  the  pipette  will  contract.  When  com- 
bustion is  complete,  the  volume  will  steadily  expand.  When 
a  decided  excess  of  oxygen  has  been  added,  close  the  clip  on 
the  pipette.  Continue  to  heat  the  spiral  for  two  or  three 
minutes  after  stopping  the  oxygen  supply. 

Pass  the  gas  into  the  burette,  and  then,  without  previous 
measurement,  into  a  potassium  hydroxide  pipette,  in  order  to 
remove  the  carbon  dioxide  formed  by  the  combustion  of  the 
methane.  Return  the  gas  to  the  burette  and  measure  the 

1  Pure  oxygen  may  be  prepared  by  heating  potassium  permanganate 
in  a  hard  glass  test-tube.  The  oxygen  is  collected  in  a  gas  holder  and 
stored  till  required.  It  should  be  analysed  and  a  correction  applied  if  it 
contains  any  nitrogen. 


268  ^  *        GAS  ANALYSIS 

volume.     Absorb  th\excess  of  oxygen  with  phosphorus  or 
sodium  hydrosulphite,  and  measure  the  residue. 

The  reactions  which  occur  in  the  combustion  are  — 


Each^volume  of  methane  requires  two  volumes  of  oxygen, 
and  evkry  two  volumes  of  hydrogen  require  one  volume  of 
oxygen.\ 

If  x  be  the  volume  of  methane,  then  the  volume  of 
oxygen  required  for  its  combustion  is  2  x. 

If  y  be  the  volume  of  hydrogen,  then  the  volume  of 

oxygen  required  for  its  combustion  is  ^. 
The     total     volume     of     oxygen     required    is    there- 
fore (zx  +  £ 

The   water   and    carbon  dioxide   are   absorbed    by    the 
reagents,  and  we  have  therefore  :  — 

(i)    The     contraction      in      volume     on     combustion 


(2)  The   volume   of    oxygen    used    in   the   combustion 


From  these  two  equations,  the  quantities  of  methane  and 
hydrogen  are  calculated. 

The  final  unabsorbed  residue  is  nitrogen,  which,  after 
correction  for  any  nitrogen  introduced  with  the  oxygen,  gives 
the  quantity  of  nitrogen  in  the  original  mixture.  This 
volume  is,  of  course,  the  same  as  that  obtained  by  subtract- 
ing the  total  of  all  the  other  constituents  from  the  original 
volume. 

There  is  no  simple  method  for  the  direct  determination 
of  nitrogen. 


Gas  Analysis  with  the  Orsat  Apparatus 

The  Orsat  apparatus  is  a  convenient,  portable  apparatus 
which  is  suitable  for  the  analysis  of  air  in  mines,  flue  gases, 
etc.  The  method  of  analysis  is  essentially  the~same  as  with 
the  Hempel  apparatus,  but  the  burette,  pipettes,  and  the 
accessories  are  so  modified  that  the  whole  apparatus, 


FIG.  67. 

including  three  or  four  pipettes,  will  fit  into  a  portable 
box.  The  number  of  pipettes  or  absorption  vessels  depends 
on  the  number  of  gases  to  be  determined.  The  Orsat 
apparatus  shown  in  Fig.  67  is  designed  for  the  determination 
of  carbon  dioxide,  oxygen,  carbon  monoxide,  and  hydrogen. 
The  apparatus  consists  essentially  of  a  measuring-tube 

269 


270 


GAS  ANALYSIS 


M,  provided  with  a  water  jacket  to  keep  the  temperature 
constant,  and  four  pipettes  A,  B,  C,  and  D. 

The  measuring-tube  M  has  a  capacity  of  100  c.c.,  with 
graduations  at  each  %  c.c.  In  order  to  economise  space,  the 
upper  (40  c.c.)  portion  of  the  tube  is  much  wider  than  the 
lower  portion.  To  the  lower  end  is  attached  a  stout  rubber 
tube  which  leads  to  the  levelling-bottle  L.  The  measuring- 
tube  terminates  at  the  upper  end  in  a  capillary  tube  T  which 
carries  four  side  tubes,  with  taps,  leading  to  the  four 
pipettes.  The  other  end  of  the  capillary  tube  T  is  fitted 
with  a  three-way  tap  W. 

Each  pipette  or  absorption  vessel  consists  of  two 
cylindrical  glass  bulbs  connected  below  by  a  bent  glass 
tube;  it  thus  resembles  a  large  U-tube  with  the  limbs 
enlarged  into  bulbs  (Fig.  68).  One  bulb  of 
each  pipette  (except  D)  is  packed  with  glass 
tubing  or  rolls  of  iron  gauze.  This  bulb  ends 
above  in  a  capillary  tube  which  is  connected  by 
a  short  piece  of  rubber  tubing  with  one  of  the 
side  tubes  of  the  capillary  T.  The  other  bulb 
of  the  pipette  is  open  to  the  air. 

In  some  forms  of  the  Orsat  apparatus,  the 
pipette  D  and  the  palladium  tube  G  are  omitted. 
For  convenience  in  description,  the  use  of  the 
simpler  form  of  apparatus  is  described  first,  and 
the  description  of  this  part  of  the  apparatus  is 
given  later. 

To  prepare  the  apparatus  for  an  analysis, 
lubricate  each  of  the  taps  with  vaseline ;  fill  the 
measuring-tube  with  water;  and  fill  the  pipettes  with  the 
appropriate  reagents.  The  reagents  used  are  the  same  as 
in  the  Hempel  pipettes  ;  fill  the  pipette  A  with  potassium 
hydroxide,  the  pipette  B  with  sodium  hydrosulphite,  and  the 
pipette  C  with  ammoniacal  cuprous  chloride. 

In  order  to  be  sure  that  the  apparatus  is  gas-tight,  close 
the  pipette  taps  and  the  tap  W,  and  place  the  levelling- 
bottle  on  the  top  of  the  box.  If  the  level  of  the  water  in 
the  measuring-tube  does  not  alter  in  the  course  of  two 
minutes,  the  apparatus  is  ready  for  an  analysis. 

Open   the   tap   W,   and   raise   the   levelling-bottle    until 


FIG.  68. 

Side  view  of 
Orsat  pipette. 


ORSAT  APPARATUS  271 

the  measuring-tube  is  filled  with  water.  Then  close  the 
tap  W,  open  the  tap  above  the  pipette  A,  and  lower  the 
levelling -bottle  so  that  the  potassium  hydroxide  solu- 
tion is  drawn  up  until  it  completely  fills  the  absorption 
bulb.  Do  not  allow  the  reagent  to  reach  the  tap,  but  close 
the  tap  when  the  liquid  reaches  the  mark  on  the  capillary 
tube  between  the  bulb  and  the  tap.  Open  the  tap  W,  and 
expel  the  air  again  from  the  measuring-tube.  In  a  similar 
manner,  remove  the  air  from  the  other  absorption  bulbs. 

Collection  of  the  Sample. — As  the  apparatus  is  portable, 
the  sample  is  drawn  directly  into  the  measuring-tube.  Close 
the  pipette  taps,  after  expelling  the  air  from  the  absorption 
bulbs.  Fill  the  measuring-tube  with  water.  Connect  one 
of  the  free  openings  of  the  tap  W  with  a  tube  leading  to 
the  flue  (or  other  source  of  the  gas),  and  connect  the  other 
opening  with  a  small  rubber  pump  P.  By  means  of  the 
pump,  draw  the  gas  to  be  analysed  through  the  leading 
tubes  until  it  has  completely  displaced  the  air.  (In  order 
to  prevent  admission  of  dust  into  the  apparatus,  it  is 
advisable  to  pass  the  incoming  gas  through  a  tube  packed 
with  cotton  wool.)  Then  turn  the  tap  W  so  as  to  bring  the 
inlet  tube  into  communication  with  the  measuring-tube. 
Lower  the  levelling-bottle,  and  allow  a  little  more  than 
100  c.c.  of  the  gas  to  enter  the  measuring-tube. 

Close  the  tap  W,  and  remove  the  connections  from  it. 
Adjust  the  position  of  the  levelling-bottle  so  that  the  surface 
of  the  water  in  it  is  level  with  the  lowest  graduation  mark. 
Open  the  tap  W  for  a  few  seconds,  and  then  close  the  tap. 
In  this  way,  100  c.c.  of  the  gas,  at  the  atmospheric  pressure 
and  at  the  temperature  of  the  water  jacket,  is  obtained. 

Analysis  of  the  Gas. — The  plan  of  operations  is  identical 
with  that  adopted  in  using  the  Hempel  apparatus.  The  gas 
is  treated  with  the  various  reagents  in  the  order  prescribed 
on  p.  266.  Carbon  dioxide  is  first  absorbed  by  means  of 
the  potassium  hydroxide  in  the  pipette  A,  as  follows  : — Open 
the  tap  above  the  pipette  A,  and  drive  the  gas  into  the 
pipette  by  raising  the  levelling-bottle.  When  the  water 
reaches  the  top  of  the  measuring-tube,  close  the  rubber 
tube  with  a  clip  at  H.  The  water  from  the  measuring- tube 
must  not  be  allowed  to  enter  the  capillary  tube  T. 


272  GAS  ANALYSIS 

On  account  of  the  large  surface  of  reagent  exposed  to 
the  gas,  absorption  is  rapid  ;  it  is  still  more  rapid  if  the 
gas  is  kept  moving  to  and  fro  between  the  pipette  and 
the  measuring-tube,  as  the  reagent  on  the  glass  tubes  in 
the  pipettes  is  thereby  constantly  renewed.  When  returning 
the  gas  to  the  measuring-tube,  take  care  that  the  reagent  does 
not  reach  the  tap  above  the  pipette. 

Allow  exactly  one  minute  for  the  water  to  drain  from  the 
side  of  the  measuring-tube,  and  read  the  volume  of  the  gas. 
Repeat  the  above  series  of  operations  until  the  volume 
remains  unaltered. 

The  absorption  of  oxygen  by  the  sodium  hydrosulphite 
in  pipette  B,  and  of  carbon  monoxide  by  the  ammoniacal 
cuprous  chloride  in  pipette  C,  is  performed  in  a  similar 
manner. 

Notes. — The  reagents  used  in  the  pipettes  must  be 
frequently  renewed,  as  they  are  not  protected  from  the 
atmosphere  as  in  a  Hempel  double  pipette. 

The  determination  of  carbon  dioxide,  in  this  as  in  all 
other  forms  of  apparatus  in  which  water  is  used  as  the 
liquid  in  the  burette,  is  inaccurate.  It  could  be  made  as 
accurate  as  the  determination  of  the  other  gases  by  using 
mercury  in  the  burette,  but  the  apparatus  would  be  no 
longer  portable. 

At  the  completion  of  a  set  of  analyses,  empty  the 
pipettes.  If  any  reagent  has  got  into  the  capillaries  or 
taps,  wash  it  out  at  once  with  water.  Lubricate  the  taps 
before  packing  away  the  apparatus. 

Determination  of  Hydrogen  by  Combustion  in 
Contact  with  Palladium. 

If  a  mixture  of  hydrogen  and  excess  of  oxygen  is  passed 
over  palladium  black,  complete  combustion  of  the  hydrogen 
takes  place.  The  reaction  is  slow  at  the  ordinary  tempera- 
ture, but  at  100°  it  is  almost  instantaneously  complete. 
Methane  and  its  homologues  are  not  burned  under  the  same 
conditions,  but  combustion  of  the  methane  becomes  appreci- 
able when  the  temperature  reaches  200°.  These  facts  form 
the  basis  of  the  following  method  for  the  determination  of 
hydrogen. 


ORSAT  APPARATUS 


273 


The  capillary  tube  G  (Fig.  67)  contains  a  thread 
of  asbestos  coated  with  palladium  black.  The  capillary 
tube  and  its  contents  can  be  heated  by  means  of  a  small 
spirit  lamp.  The  method  of  using  the  apparatus  is  as 
follows  : — 

Fill  the  bulb  D  with  water,  expelling  the  air  as  described 
above.  Determine  the  carbon  dioxide,  oxygen,  and  carbon 
monoxide.  Then  lower  the  levelling-bottle,  open  the  tap 
W,  allow  air  to  enter  until  the  total  volume  is  nearly 
100  c.c.,  and  close  the  tap  W.  Read  the  volume  accurately. 
Heat  the  capillary  G  gently  by  means  of  a  small  flame,  and 
pass  the  gas  very  slowly  from  the  measuring-tube  into  the 
pipette  D.  If  hydrogen  is  present  in  quantity,  the  first 
portion  of  the  palladinised  asbestos  will  glow  on  account  of 
the  heat  liberated  by  the  combustion.  The  glowing  portion 
should  not  be  more  than  i  mm.  in  length. 

Return  the  gas  to  the  burette,  and  measure  the  volume. 
Any  trace  of  carbon  monoxide  is  also  burned.  (Usually  a 
trace  of  the  carbon  monoxide  escapes  absorption  by  the 
cuprous  chloride  in  this  apparatus.)  If  any  methane  is 
present  a  minute  fraction  of  it  will  also  be  burned.  Remove 
any  carbon  dioxide,  therefore,  by  means  of  potassium 
hydroxide,  and  again  measure  the  volume. 

Two  volumes  of  hydrogen  require  one  volume  of  oxygen 
for  combustion,  so  that  two-thirds  of  the  total  contraction 
represents  the  volume  of  hydrogen.  If  any  carbon  dioxide  is 
found  after  the  combustion,  a  correction  must  be  applied. 
The  following  example  of  an  analysis  shows  the  method  of 
calculation : — 


Initial  reading        .        ,  .  100  c.c. 

After  potassium  hydroxide  .  91-2  c.c. 

After  sodium  hydrosulphite  .  83*2  c.c. 

After  cuprous  chloride    .  .  77-1  c.c. 


Carbon  dioxide 
Oxygen  .        .        . 
Carbon  monoxide  . 


After  addition  of  air        .        .     99-4  c.c. 

After  burning  the  mixture       .     89-8  c.c.  [-Hydrogen 

After  removing  carbon  dioxide     89.2  c.c. 


Per  Cent. 

8-8 

8-0 
6-1 


6-0 


274  GAS  ANALYSIS 

It  is  not  possible  satisfactorily  to  correct  for  the  carbon 
dioxide  found  after  the  combustion,  if  both  carbon  monoxide 
and  methane  are  present  in  the  mixture.  The  matter  is 
not  of  any  practical  importance  as  the  Orsat  method  is  not 
capable  of  yielding  very  accurate  results ;  a  rough  correction 
will  make  any  error  from  this  source  smaller  than  other 
errors  inherent  in  the  method.  When  carbon  monoxide  is 
burned,  the  contraction  is  equal  to  half  the  volume  of  the 
carbon  dioxide  produced,  whereas,  when  methane  is  burned, 
the  contraction  is  equal  to  twice  the  volume  of  the  carbon 
dioxide  produced.  The  assumption  is  therefore  made  that 
the  gas  yielding  carbon  dioxide  was  partly  methane  and 
partly  carbon  monoxide,  and  that  the  contraction  on  its 
combustion  is  equal  to  the  volume  of  the  carbon  dioxide 
produced. 

The  total  contraction  on  combustion  (after  removal  of  the 
carbon  dioxide)  was  10-2  c.c.  Of  this,  06  c.c.  was  carbon 
dioxide.  The  contraction  due  to  the  combustion  of  the 
hydrogen  was  therefore : 

10.2  —  (0.6  x  2)  =  9.0  c.c. 

Of  this  9-0  c.c.,  two-thirds  were  hydrogen  and  one-third 
oxygen ;  the  volume  of  hydrogen  present  was  therefore 
6-0  c.c. 


Analyses  Involving  the  Use  of  a 
Lunge  Nitrometer 

The  Lunge  nitrometer  (Fig.  69)  consists  of  two  glass 
tubes  connected  by  a  stout  rubber  tube.  The  levelling-tube 
L  is  ungraduated  and  is  open  to  the  air;  the  measuring-tube 
M  is  graduated  and  is  fitted  at  the  top  with  a  three-way  tap, 
so  that  the  measuring-tube  may  be  connected  with  either 
the  cup  C  or  the  tube  A.  By  means  of  this  tap  a  sample  of 
gas  may  be  drawn  into  the  apparatus  through  the  tube  A, 
and  then,  by  turning  the  tap,  any  desired  reagent  may  be 
run  in  from  the  cup  C.  The  pressure  tubing  used  to  connect 
the  two  glass  tubes  should  be  a  little  longer  than  the 
measuring-tube,  and  must  be  securely  wired  on  to  the  tubes. 

At  all  times  the  apparatus  should  stand  on  a  mercury 
tray.  Special  clamps  with  wide  grips  may  be  obtained  for 
holding  the  apparatus,  but  good  burette  clamps,  with  rubber 
pads  on  the  grips,  are  quite  satisfactory.  Each  tube,  when 
filled  with  mercury,  weighs  about  \\  kilos;  the  use  of  a 
flimsy  burette  clamp,  particularly  if  the  tightening-screw  has 
a  coarse  thread,  means  the  almost  certain  breakage  of  the 
apparatus. 

Nitrogen  in  a  Nitrate  or  Nitrite. 

OUTLINE  OF  METHOD. — The  nitrate  or  nitrite  is  reduced  to  nitric  oxide 
by  shaking  with  mercury  and  concentrated  sulphuric  acid  in  the 
measuring-tube  of  a  Lunge  nitrometer.  The  nitric  oxide  is 
measured,  and,  from  the  volume  of  nitric  oxide,  the  weight  of 
nitrogen  is  calculated. 

Procedure. — The  solution  containing  the  nitrate  should 
not  exceed  2  c.c.  in  volume ;  if  necessary,  it  must  be  eva- 
porated until  reduced  to  this  volume. 

Completely  fill  the  measuring-tube  of  the  nitrometer 
(Fig.  69)  with  mercury  and  pour  the  nitrate  solution  into 
the  cup  C.  By  cautiously  opening  the  tap,  run  the  solution 

275 


276 


GAS  ANALYSIS 


A 


into  the  nitrometer  without  admission  of  any  air.  Wash  out 
the  vessel  which  contained  the  nitrate  with  a  few  drops  of 
dilute  sulphuric  acid,  and  run  this  in 
through  the  cup  in  a  similar  manner. 
(Keep  the  volume  of  liquid  and  wash- 
ings as  small  as  possible  —  the  total 
should  not  in  any  case  exceed  4  c.c.). 
Pour  about  15  c.c.  of  concentrated  sul- 
phuric acid  into  the  cup  and  run  this 
into  the  nitrometer,  care  being  taken 
that  no  air  is  admitted.  Remove  the 
measuring-tube  from  the  clamp  and 
shake  at  once,  with  a  rotatory  motion, 
so  that  globules  of  mercury  are  thrown 
up  into  the  liquid.  In  a  minute  or 
two  the  evolution  of  nitric  oxide  will 
begin.  The  shaking  must  be  continued 
until  the  evolution  of  nitric  oxide  ceases. 
Set  the  apparatus  in  a  cool  place  until 
it  has  attained  the  room  temperature, 
adjust  the  pressure  by  means  of  the 
levelling-tube,  and  measure  the  volume 
of  the  gas. 

When    adjusting    the   pressure,   the 
density  of  the  aqueous   liquid  may  be 


FIG.  69. 


taken  as  one-eighth  that  of  mercury.  Care  must  be  taken 
not  to  warm  the  gas  by  handling  the  tube  when  adjusting  the 
pressure.  The  temperature  of  the  room  and  the  barometric 
pressure  are  also  required. 

One  c.c.  of  nitric  oxide  at  o°  and  760  mm.  weighs 
0-001341  gram,  and  corresponds  to  0-000626  gram  of 
nitrogen. 

From  the  volume  of  nitric  oxide,  corrected  to  o°  and 
760  mm.,  calculate  the  weight  of  nitrogen. 

Exercise, — Determine  the  percentage  of  nitrogen  in 
potassium  nitrate.  Use  about  0-2  gram. 


LUNGE  NITROMETER 


277 


Hydrogen  Peroxide. 

Hydrogen  peroxide,  in  presence  of  sulphuric  acid,  inter- 
acts quantitatively  with  potassium  permanganate,  according 
to  the  equation — 
5H,O,  +  2KMnO4  +  4H2SO4  =  2KHSO4  +  2MnSO4  +  $O2  +  8H2O. 

The  concentration  of  a  solution  of  hydrogen  peroxide  may 
therefore  be  determined  by  measure- 
ment of  the  volume  of  oxygen  obtained 
from  a  known  volume  of  the  peroxide 
solution  on  treatment  with  excess  of 
permanganate. 

The  apparatus  required  consists  of  a 
reaction  vessel,  shown  in  Fig.  70,  to- 
gether with  a  Lunge  nitrometer.  (A 
Hempel  gas  burette  may  be  used  instead 
of  the  nitrometer,  but  is  not  so  con- 
venient.) The  reaction  vessel  consists 
of  a  small  conical  flask  with  a  well-fitting 
rubber  cork  which  carries  a  glass  tube. 
Connection  with  the  tube  A  of  a  Lunge 
nitrometer  is  made  by  means  of  a  piece 
of  pressure  tubing.  A  short  wide  test- 
tube  is  also  required ;  this  must  be 
slightly  longer  than  the  base  of  the  flask  in  order  that,  when 
placed  in  the  flask,  it  will  remain  in  the  position  shown 
in  the  diagram. 

Procedure. — Place  20  c.c.  of  saturated  potassium  per- 
manganate solution  and  20  c.c.  of  dilute  sulphuric  acid  in  the 
conical  flask.  Dilute  10  c.c.  of  commercial  hydrogen  peroxide 
to  100  c.c.,  and  place  10  c.c.  of  this  diluted  solution  in  the 
test-tube.  Place  the  test-tube  carefully  in  the  flask,  care 
being  taken  that  no  mixing  of  the  peroxide  and  permanganate 
occurs  at  this  stage. 

Insert  the  rubber  cork  and  connect  the  flask  with  the  nitro- 
meter, which  is  filled  with  mercury.  Loosen  the  tap  of  the 
nitrometer  in  its  socket,  and  place  the  conical  flask,  up  to  the 
neck,  in  water  at  the  room  temperature. 

After  about  five  minutes,  adjust  the  mercury  to  the  zero 
mark,  insert  the  tap  firmly  into  position,  and  turn  it  so  that 


FIG.  70. 


278  GAS  ANALYSIS 

the  reaction  vessel  is  in  communication  with  the  measuring- 
tube.  If  the  mercury  level  alters  in  the  course  of  a  few 
minutes,  the  temperature  is  not  yet  constant. 

When  the  temperature  has  become  constant,  tilt  the 
reaction  flask  so  that  the  peroxide  solution  mixes  with  the 
permanganate.  Keep  the  pressure  approximately  equal  to 
the  atmospheric  pressure  by  lowering  the  levelling-tube  from 
time  to  time.  Rinse  out  the  small  tube  with  some  of  the 
permanganate  solution  by  appropriate  manipulation  of  the 
flask.  When  the  reaction  is  apparently  complete,  shake  the 
flask  vigorously.  (The  liquid,  unless  violently  agitated,  may 
retain  several  cubic  centimetres  of  dissolved  oxygen.) 

Adjust  the  mercury  levels  in  the  two  tubes,  and  read  the 
volume.  From  the  volume  of  oxygen,  corrected  to  o°  and 
760  mm.,  calculate  the  weight  of  hydrogen  peroxide  per  litre 
in  the  original  solution. 

It  is  a  common  practice  to  express  the  concentration  of 
hydrogen  peroxide  in  terms  of  the  volume  of  oxygen  obtained 
on  treatment  with  permanganate.  Thus,  "five  volume" 
hydrogen  peroxide  solution  yields  five  times  its  own  volume 
of  oxygen  at  room  temperature.  (It  may  be  pointed  out 
that  half  of  the  oxygen  comes  from  the  permanganate.) 
Calculate  the  concentration  of  the  peroxide  solution  in  this 
way  also. 

Valuation  of  Zinc  Dust. 

Commercial  zinc  dust  is  always  contaminated  with  zinc 
oxide,  together  with  small  amounts  of  iron  and  other  metals. 
It  is  largely  used  as  a  reducing  agent,  and  an  estimate  of  its 
value  for  this  purpose  may  be  obtained  by  measuring  the 
volume  of  hydrogen  evolved  when  a  weighed  sample  is  treated 
with  excess  of  acid. 

The  apparatus  (Fig.  70)  used  for  the  determination  of 
hydrogen  peroxide  is  suitable  for  this  analysis  also. 

Place  about  o-i  gram  (accurately  weighed)  of  the  zinc 
dust  in  the  conical  flask,  and  add  about  25  c.c.  of  water.  In 
the  small  test-tube  place  5  c.c.  of  concentrated  sulphuric  acid. 
The  further  procedure  is  the  same  as  in  the  determination  of 
hydrogen  peroxide. 

From  the  volume  of  hydrogen  liberated,  calculate  the 
percentage  of  metallic  zinc  in  the  zinc  dust. 


Determination  of  Gases  present 
only  in  Traces 

Methods  depending  on  the  alteration  in  volume  produced 
by  absorption  are  not  suitable  for  the  determination  of  traces 
of  gases.  An  accuracy  of  i  in  1000  in  the  measurement  of  a 
gas-volume  is  within  the  limits  of  what  one  would  ordinarily 
regard  as  "  permissible  error."  An  illustration  will  make  it 
clear  that  this  error  is  far  too  large  for  many  purposes.  The 
amount  of  carbon  dioxide  in  the  atmosphere  is  usually  about 
3  parts  in  10,000  or  0-03  per  cent,  and  0-03  c.c.  would  be 
the  alteration  in  volume  produced  by  absorption  of  the  carbon 
dioxide  in  100  c.c.  of  air.  Even  assuming  that,  by  special 
precautions,  the  error  of  measurement  was  reduced  to  I  in 
10,000,  there  would  still  be  an  uncertainty  of  about  30  per 
cent,  in  the  amount  of  carbon  dioxide. 

In  practice,  therefore,  traces  of  gases  are  determined  in  a 
different  manner.  A  large  volume  of  the  gas  mixture  is 
treated  with  a  suitable  absorbent,  and  the  absorbed  con- 
stituent is  then  determined  by  analysis  of  the  reagent.  For 
example,  carbon  dioxide  in  air  may  be  determined  by  treat- 
ing a  large  measured  volume  of  air  with  a  measured  volume 
of  standard  baryta  solution,  and  finding,  by  analysis,  how 
much  of  the  baryta  has  been  converted  into  carbonate.  The 
attainable  accuracy  is  obviously  greatly  increased  by  the 
substitution  of  a  chemical  determination  of  the  carbonate 
for  the  measurement  of  a  minute  alteration  in  a  large 
volume.  A  comparatively  rough  measurement  of  the  total 
volume  is  usually  sufficiently  accurate  for  a  process  of  this 
kind. 

Measurement  of  the  Gas  Mixture. — From  the  nature  of 
the  case,  no  general  rules  can  be  laid  down.  For  the  measure- 
ment of  coal  gas  or  other  gas  of  which  a  large  supply  is 
available,  a  gas  meter  is  most  convenient.  The  special 

279 


280 


GAS  ANALYSIS 


description  of  the  determination  of  carbon  dioxide  in  air 
(see  p.  282)  may  suggest  a  method  which  could  be  adapted 
to  other  cases. 

Absorption  Apparatus. — Convenient  forms  of  apparatus 
for  holding  the  absorption  reagent  are  shown  in  Fig.  71, 
A  and  B.  If  the  gas  contains  dust,  it  is  necessary  to  pass 


FIG.  71. 

it   through   a  tube    packed   with    pumice    or    glass    beads, 
moistened  with  the  reagent  (as  in  Fig.  71,  C). 

Sulphur  in  Coal  Gas. 

OUTLINE  OF  METHOD. — The  coal  gas  is  burned  in  air  and  the  sulphur 

-  dioxide  formed  is  absorbed  by  a  solution  of  sodium  carbonate  and 

bromine.     All  the  sulphur  is  thereby  obtained  as  sodium  sulphate, 

the   sulphate  being   determined  gravimetrically   in    the    ordinary 

manner. 

The  gas  is  measured  by  a  gas  meter  and  is  led  by  the 
tube  A  into  the  glass  flask  B  (Fig.  72).  This  is  a  i  litre, 
round-bottomed,  hard-glass  flask  with  a  short  wide  mouth. 
The  inlet  tube  must  be  of  hard  glass,  and  is  drawn  to  a 
fine  jet  at  C  where  the  gas  is  burned.  The  air  required  for 
the  combustion  of  the  gas  is  freed  from  any  traces  of 


SULPHUR  IN  COAL  GAS 


281 


hydrogen  sulphide  in  the  laboratory  atmosphere  by  passing 
it  through  the  purifier  E  filled  with  pumice,  upon  which  a 
concentrated  solution  of  potassium  hydroxide  is  constantly 
dropping  from  a  tap  funnel.  The  purified  air  then  passes 


FIG.  72. 

by  the  tube  D  into  the  flask  B.  The  products  of  the  com- 
bustion are  drawn  out  of  the  flask  by  means  of  a  filter- 
pump  through  the  tube  F,  but,  before  reaching  the  pump, 
pass  through  three  wash-bottles  G,  H,  K,  in  which  the 
sulphur  dioxide  is  retained.  The  wash-bottles  each  contain 
sodium  carbonate  solution.  To  the  first  two,  G  and  H,  a 


282  GAS  ANALYSIS 

few  drops  of  bromine  are  also  added  in  order  to  oxidise  the 
sulphite  to  sulphate. 

A  larger,  steady  flame  may  be  obtained  by  the  modified 
arrangement  of  the  gas  and  air  supplies  shown  at  the  side  of 
the  diagram.  The  gas  is  led  in  through  the  central  tube 
and  the  air  through  the  outer  tube. 

Procedure. — Pass  the  coal  gas  through  the  meter  for  a 
few  minutes,  and  draw  a  rapid  current  of  air  through  the 
apparatus  by  means  of  a  filter-pump.  Withdraw  the  cork 
carrying  the  three  tubes  from  the  flask  B,  and  ignite  the  gas 
at  C.  By  means  of  the  screw-clip  N,  cut  down  the  gas  supply 
until  the  flame  is  about  10  mm.  high ;  then  insert  the  cork 
in  the  flask.  By  regulation  of  the  air  and  gas  supplies,  adjust 
the  flame  so  that  it  burns  with  sharply  defined  edges. 

When  about  50  litres  of  gas  have  been  burned,  cut  off  the 
gas  supply.  Wash  the  contents  of  the  wash-bottles  G  and  H 
into  a  beaker,  and  rinse  the  flask  B  into  the  same  beaker. 
Acidify  the  solution  with  hydrochloric  acid,  boil  until  the 
excess  of  bromine  is  expelled,  and  determine  the  sulphate 
gravimetrically  as  barium  sulphate. 

It  is  advisable  to  test  the  bromine  used,  as  it  sometimes 
contains  sulphuric  acid. 

Atmospheric  Carbon  Dioxide. 

OUTLINE  OF  METHOD. — The  sample  of  air,  contained  in  a  dry  bottle  of 
known  capacity,  is  shaken  with  a  measured  volume  of  standard 
baryta  solution  until  the  absorption  of  the  carbon  dioxide  is  complete. 
The  precipitated  barium  carbonate  is  removed  by  filtration,  and  the 
excess  of  barium  hydroxide  is  determined  by  titration  with  standard 
hydrochloric  acid,  these  operations  being  conducted  in  such  a  way 
that  the  baryta  is  protected  from  expired  air.  The  amount  of 
carbon  dioxide  is  calculated  from  the  amount  of  baryta  transformed 
into  carbonate. 

The  following  solutions  are  required  : — 

Decinormal  Hydrochloric  Acid. — Dilute  10  c.c.  of  con- 
centrated hydrochloric  acid  to  I  litre  and  standardise  by 
means  of  calcite,  or  by  titration  with  standard  baryta. 

Fiftieth-normal  Baryta. — Prepare  as  described  on  p.  61. 

The  Apparatus  required  is  shown  in  Fig.  73.  The 
absorption  bottle  C,  of  which  several  should  be  provided,  is 


ATMOSPHERIC  CARBON  DIOXIDE 


283 


a  Winchester  quart,  the  capacity  of  which  has  been  previously 
determined.  It  is  furnished  with  a  rubber  stopper,  through 
which  pass  two  tubes,  the  shorter 
tube  being  flush  with  the  stopper 
inside  the  bottle.  The  tubes  pro- 
ject 4  or  5  inches  externally,  and 
are  provided  with  taps.  The  shorter 
tube  is  connected  by  means  of  a 
piece  of  narrow-bore  rubber  tubing, 
8  inches  long,  with  a  filtering-tube 
(Fig.  73,  F)  containing  an  asbestos 
filter.  This  tube  is  fitted  into  a 
200  c.c.  filter-flask  by  means  of  a 
rubber  stopper.  The  filter-flask 
and  the  absorption  bottle  are  held 
in  clamps  fixed  to  a  retort  stand. 

Collecting  the  Sample.  —  By 
means  of  bellows  or  a  suction 
pump,  fill  the  bottle  with  the  air 
to  be  analysed,  care  being  taken 
that  expired  air  is  not  drawn 
directly  into  the  bottle.  Close 


FIG.  73. 


the  bottle  with  the  rubber  stopper  and  tubes  (see  above), 
or  with  a  glass  stopper  smeared  with  a  trace  of  grease. 
Note  the  temperature  at  the  time  of  collection.  In 
ordinary  practice  it  is  unnecessary  to  note  the  barometric 
pressure,  as  the  error  which  may  be  introduced  by  assuming 
that  the  pressure  is  760  mm.  is  less  than  other  errors 
inherent  in  the  method. 

Procedure. — If  the  absorption  bottle  has  been  closed 
with  a  glass  stopper,  replace  the  latter  quickly  in  the  open 
air  by  the  rubber  stopper  and  tubes,  just  before  the  analysis 
is  carried  out. 

Insert  the  jet  of  the  baryta  burette  into  the  longer  tube, 
open  the  taps  A  and  B,  and  run  5<Dc.c.  of  baryta  into  the  bottle. 
Close  the  taps,  wet  the  whole  interior  of  the  bottle  with  the 
baryta  solution,  lay  the  bottle  on  its  side,  and  shake  it  at 
frequent  intervals  during  fifteen  or  twenty  minutes. 

Meanwhile,  prepare  the  asbestos  filter  by  pushing  a 
small  plug  of  glass  wool  into  the  filtering-tube,  and  then 


284  GAS  ANALYSIS 

pour  into  the  tube  a  small  quantity  of  the  asbestos  mixture 
used  for  Gooch  crucibles.  Connect  the  filter-flask  with  the 
pump  and  wash  the  filter  several  times  with  water.  Then 
empty  the  filter-flask  and  measure  into  it  10  c.c.  of  the 
standard  hydrochloric  acid.  Prepare  also  a  special  solution 
for  washing  out  the  absorption  bottle,  by  adding  to  100  c.c. 
of  distilled  water  (which  always  contains  carbon  dioxide) 
2  c.c.  of  phenolphthalein,  a  little  barium  chloride,  and  then 
baryta  solution  until  an  incipient  pink  colour  is  produced. 
The  purpose  of  the  barium  chloride  is  to  diminish  the  solvent 
action  of  the  water  on  the  barium  carbonate. 

When  the  absorption  is  complete,  fix  the  bottle  in  its 
clamp  and  connect  with  the  filtering-tube.  Start  the  filter- 
pump  at  a  moderate  speed  and  open  tap  A.  The  barium 
carbonate  remains  on  the  asbestos,  and  the  clear  baryta 
which  passes  through  is  at  once  neutralised  by  the  hydro- 
chloric acid.  Leave  A  open  for  about  thirty  seconds  after 
the  baryta  has  passed  through  the  filter  in  order  to  exhaust 
the  bottle  partially,  then  close  A. 

Dip  the  end  of  tube  B  into  the  special  washing  solution 
contained  in  a  small  beaker,  open  B  carefully,  and  allow 
about  20  c.c.  to  pass  into  the  bottle,  then  close  B.  Remove 
the  bottle  from  its  clamp  and,  by  rotating  and  shaking  whilst 
in  a  horizontal  position,  thoroughly  rinse  the  whole  interior 
of  the  bottle,  replace  it  in  the  clamp,  allow  the  liquid  to  drain 
for  a  few  seconds,  and  again  open  A. 

Repeat  these  operations  several  times  until  the  absence 
of  colour  in  the  washings  shows  that  the  baryta  has  been 
completely  removed  from  the  bottle. 

Now  admit  air  to  the  apparatus,  detach  the  filter-flask, 
pour  the  contents  into  a  porcelain  basin,  and  titrate  with 
baryta  until  the  solution  is  nearly  neutral.  Wash  out  the 
filter-flask  with  a  portion  of  the  nearly  neutral  liquid,  and 
finish  the  titration. 

Example — 

i  c.c.  baryta  =  0-2201  c.c.  of  CO2  at  N.T.P. 

10  c.c.  hydrochloric  acid  =    49-0  c.c.  baryta. 
Capacity  of  bottle  =     2650  c.c. 

Fifty  c.c.  of  baryta  were  placed  in  the  bottle,  and   10  c.c.  of 


ATMOSPHERIC  CARBON  DIOXIDE  285 

hydrochloric  acid  in  the  filter-flask.  After  absorption  of  the 
carbon  dioxide,  the  acid  remaining  in  the  filter-flask  required 
3-0  c.c.  of  baryta  to  neutralise  it. 

Total  baryta  used  =  50  + 3  =  53  c.c. 

But  10  c.c.  acid  neutralises  49-0  c.c.  baryta. 

Therefore,  the  CO2  neutralised  53  —  49  =  4  c.c.  baryta. 

The  volume  of  air  was  equal  to  2650—50=2600  c.c.,  and  the 
temperature  was  15°  C. 

The  amount  of  carbon  dioxide  in  the  sample  of  air  was 
therefore : — 

4-0x0.2201x10,540    =     5;  vols  of  co  in  I0)000  vok  of  ain 

2600 

[10,540  volumes  at  15°  are  equal  to  10,000  volumes  at  o°.] 

Hydrogen  Sulphide  in  Coal  Gas. 

After  measurement,  the  gas  is  dried  by  passing  over 
calcium  chloride,  and  the  hydrogen  sulphide  is  then 
absorbed  in  U -tubes,  packed  with  dehydrated  copper  sulphate. 
Each  u-tube  should  be  almost  filled  with  granulated  pumice 
impregnated  with  copper  sulphate,  and  a  little  calcium 
chloride  placed  at  the  exit  end.  The  pumice  is  prepared 
for  use  by  soaking  it  in  a  hot  saturated  copper  sulphate 
solution  and  then  drying  it  for  four  hours  at  150°  to  160°. 

The  increase  in  weight  of  the  U-tubes  gives  the  amount  of 
hydrogen  sulphide. 

Hydrocyanic  Acid  in  Coal  Gas. 

From  50  to  100  litres  of  the  gas  are  passed  through  a 
meter,  and  then  through  several  tubes  filled  with  concentrated 
sodium  hydroxide  solution.  The  amount  of  cyanide  is  then 
determined  volumetrically  (p.  99). 

Sulphur  Dioxide  in  Flue  Gases. 

Some  special  form  of  apparatus  for  the  measurement  of 
the  gas  or  for  the  collection  of  a  sample  is  here  necessary. 
The  sulphur  dioxide  is  absorbed  by  a  solution  of  sodium 
carbonate  and  bromine  (see  under  the  determination  of 
sulphur  in  coal  gas),  and  the  sulphate  produced  is  deter- 
mined gravimetrically. 


PART   VIII 
WATER  ANALYSIS 

Pure  and  Natural  Water. — Natural  water  is  never  ideally 
pure  water.  It  always  contains,  in  solution  or  suspension, 
inorganic  and  organic  substances  derived  from  the  air,  the 
soil,  and  the  rocks  with  which  it  comes  into  contact ;  and  it 
may  have  become  polluted,  directly  or  indirectly,  with 
sewage  or  other  waste  products  of  human  life  and 
industry. 

The  substances  which  may  be  present  in  a  natural  water 
are  chiefly  (i)  calcium,  magnesium,  and  sodium  salts,  in 
the  form  of  carbonate,  chloride,  sulphate,  and  nitrate;  (2) 
comparatively  small  amounts,  or  traces,  of  iron  and  ammonium 
salts,  nitrite,  and  phosphate;  (3)  dissolved  gases — carbon 
dioxide,  oxygen,  and  nitrogen ;  (4)  dead  organic  matter,  in 
solution  or  suspension;  (5)  living  organisms,  including 
bacteria. 

Purpose  of  Water  Analysis. — An  examination  of  a  water 
supply  is  generally  made  for  the  purpose  of  deciding  whether 
the  water  is  suitable  for  domestic  or  industrial  purposes.  In 
the  case  of  a  potable  water,  it  is  necessary  to  ascertain  that 
the  salts  present  are  not  objectionable,  either  as  regards  their 
nature  or  their  amount,  and  that  the  water  does  not  contain 
more  than  a  mere  trace  of  organic  matter. 

For  washing  and  scouring,  it  is  desirable  to  use  what  is 
familiarly  known  as  a  "  soft "  water,  i.e.,  a  water  containing 
comparatively  small  amounts  of  calcium  and  magnesium 
salts.  For  steam-raising  purposes,  a  soft,  alkaline  water, 
free  from  substances  that  produce  hard  scale,  corrosion,  or 
priming,  has  many  advantages ;  and  many  important 
industries,  such  as  brewing,  paper-making,  and  dyeing,  are 
dependent  upon  a  satisfactory  water  supply.  The  water 

286 


WATER  ANALYSIS  287 

best  adapted  for  domestic  use  is  equally  suitable  for  many 
technical  purposes ;  but  the  presence  of  certain  salts  may  be 
desirable  for  one  manufacturing  process  and  objectionable  for 
another. 

Methods  of  Examination. 

The  investigation  of  a  water  supply  includes  an  examina- 
tion of  (i)  the  source  from  which  the  water  is  derived;  and 
(2)  the  water  itself.  The  two  main  sources  of  water  suitable 
for  general  purposes  are  : — 

(a)  Ground  Water,  i.e.,  wells  and  springs. 

(b)  Surface  Water,  i.e.,  streams  and  rivers. 

The  Examination  of  the  Source  of  the  supply  is  directed 
mainly  to  the  detection  of  pollution  or  of  possible  sources  of 
pollution.  The  importance  of  this  examination  lies  in  the 
fact  that  a  water  supply,  although  satisfactory  as  a  rule,  may 
be  liable  to  occasional  pollution  which  an  analysis  of  the  water 
itself  may  fail  to  disclose.  A  careful  inspection  of  the  source 
may,  in  fact,  render  a  complete  analysis  of  the  water  almost 
superfluous,  and  it  will,  in  any  case,  assist  in  the  correct 
interpretation  of  the  analytical  results. 

Shallow  wells  are  often  liable  to  pollution,  on  account 
of  either  their  position  or  their  unsatisfactory  construction. 
It  may  be  possible  to  remove  the  source  of  pollution,  or  to 
prevent  the  impurity  reaching  the  well.  In  the  case  of 
river  water,  pollution — either  from  sewage  or  other  waste 
products  draining  into  the  river,  or  from  cultivated  land 
through  which  the  river  flows — is  almost  inevitable,  and 
river  water  must  always  be  purified  by  storage  and  efficient 
filtration  before  it  can  form  a  satisfactory  water  supply  for 
household  purposes. 

The  Examination  of  the  Water  comprises  (i)  a  physical 
and  chemical  examination  ;  (2)  a  microscopical  examination ; 
and  (3)  a  bacteriological  examination. 

Provided  the  source  of  the  supply  is  satisfactory,  the 
appearance  of  the  water  and  a  comparatively  simple  chemical 
examination  will,  in  some  cases,  suffice  to  indicate  whether 
a  water  is  suitable  for  general  purposes;  but,  in  order  to 
determine  whether  a  water  is  fit  for  drinking  or  domestic 
use,  a  microscopical  and  bacteriological  examination  can 


288  WATER  ANALYSIS 

rarely  be  dispensed  with.  It  is  usually  necessary  to  examine 
the  water  more  especially  for  pathogenic  bacteria — regarding 
which  a  chemical  analysis  affords  no  information — and,  if 
sewage  pollution  is  at  all  possible,  a  bacteriological  examina- 
tion is  practically  imperative.  Turbid  water  and  water 
containing  visible  suspended  particles  should  also  be 
examined  microscopically.  In  what  follows,  only  the 
physical  and  chemical  methods  of  examination  will  be 
considered. 


Physical    and  Chemical    Methods   of 
Examination  and   Analysis 

When  a  water  supply  is  intended  for  a  drinking  water, 
it  is  usually  desirable  to  make  a  comprehensive  investigation, 
involving  most  of  the  qualitative  and  quantitative  deter- 
minations described  below.  If  the  water  is  to  be  used  for 
industrial  purposes,  a  more  restricted  examination  will 
generally  suffice — the  chief  constituents  of  importance  in 
that  case  being  the  bicarbonates,  sulphates,  and  chlorides  of 
calcium  and  magnesium.  In  special  cases,  the  determination 
of  an  accidental  impurity  may  be  of  importance. 

The  results  of  the  quantitative  determinations  are  usually 
expressed  in  parts  per  100,000  of  water.1 

COLLECTION   OP   SAMPLES   OP   WATER. 

At  least  2  litres  of  water  are  required  for  a  fairly 
complete  analysis.  Samples  are  most  conveniently  collected 
in  Winchesters.  The  bottles  should  be  of  light-coloured  glass, 
and  those  which  originally  contained  concentrated  acids 
should  be  selected.  Before  being  used  for  samples,  the 
bottles  must  be  carefully  rinsed  with  water,  and  should  be 
filled  completely  with  the  rinsing  water  at  least  once. 
Bottles  intended  for  holding  water  samples  should  be 
reserved  for  that  purpose  alone.  For  convenience  and 
safety  during  transport,  the  bottles  may  be  placed  upright 
in  wicker  baskets  or  in  padded  boxes  made  for  the  purpose. 

If  the  sample  is  to  be  collected  from  a  tap,  the  water 
should  be  allowed  to  run  to  waste  for  several  minutes  before 
filling  the  bottle.  In  taking  a  sample  from  a  river,  loch, 

1  Results  are  sometimes  given  in  grains  per  gallon  ;  to  convert  them 
into  parts  per  100,000,  divide  by  07.  It  may  also  be  noted  that 
milligrams  per  100  c.c.  correspond  to  parts  per  100,000. 

289  T 


£90  WATER  ANALYSIS 

cistern,  etc.,  the  closed  bottle  should  be  wholly  immersed  in 
the  water,  and  the  stopper  then  withdrawn.  Before  the 
bottle  is  filled,  it  should  be  rinsed  out  several  times  with  the 
water.  The  bottle  is  then  completely  filled,  a  little  water  is 
poured  away,  and  the  clean  stopper — which  should  also  be 
rinsed  with  the  water — is  firmly  inserted.  It  is  sometimes 
advisable  to  secure  the  stopper  in  its  place  by  tying  over  it 
a  piece  of  clean  linen  cloth.  The  bottles  containing  the 
samples  should  be  labelled  with  a  distinctive  number  or 
letter,  and  a  note  made  of  the  exact  place  where  each  sample 
was  taken,  with  the  hour  and  date  of  collection. 

An  analysis  of  a  single  sample  affords  information 
regarding  the  state  of  the  water  only  at  the  time  the  sample 
was  taken,  and,  as  many  waters  vary  in  character  from  time 
to  time,  it  is  often  necessary  to  make  a  periodic  examination. 
After  heavy  rain,  for  example,  some  waters  are  often  turbid  ; 
and  pollution  is  frequently  intermittent.  An  endeavour 
should  be  made  to  obtain  samples,  not  only  when  the  water 
is  in  its  normal  state,  but  also  when  its  condition  is  abnormal. 

The  examination  of  the  water  should  be  commenced  as 
soon  as  possible  after  the  sample  was  collected. 

PHYSICAL   EXAMINATION. 

The  physical  examination  includes  the  colour,  turbidity, 
odour,  and  taste  of  the  water,  and  the  determination  of  its 
electrical  conductivity. 

Odour.  —  When  the  bottle  is  first  opened,  ascertain 
whether  the  water  has  any  odour.  If  it  has  little  or  none, 
warm  100  c.c.  to  about  50°  in  a  clean  flask,  shake  briskly, 
and  try  again.  Good  water  has  no  smell,  even  when  warmed. 

Turbidity. — Examine  the  water  contained  in  a  large 
flask  or  bottle  of  colourless  glass,  and  note  its  general 
appearance,  i.e.,  whether  it  is  bright  and  perfectly  clear, 
or  whether  it  is  opalescent  or  turbid.  If  the  turbidity  is 
very  pronounced,  it  may  be  desirable  to  ascertain  approxi- 
mately the  amount  of  suspended  matter. 

Filter  a  measured  volume  (100  to  1000  c.c.  according  to 
the  degree  of  turbidity)  through  a  tared  Gooch  crucible 
containing  a  thick  pad  of  asbestos.  Dry  the  crucible  and 


PHYSICAL  EXAMINATION  291 

contents  in  the  steam-oven,  cool,  and  weigh.     Express  the 
result  in  parts  per  100,000. 

Taste. — It  is  inadvisable  to  taste  water  samples  indis- 
criminately, and,  in  any  case,  taste  and  smell  are  often 
difficult  to  differentiate.  The  presence  of  iron,  salt,  peat,  etc., 
may  confer  on  water  a  characteristic  taste,  but  the  presence 
of  these  substances  can  be  ascertained  in  other  ways.  Pure 
water,  properly  aerated,  may  be  described  as  tasteless. 

Colour. — Special  instruments  are  often  used  to  measure 
the  colour  of  water,  but  for  most  purposes  it  is  sufficient  to 
note  the  colour  (and  turbidity)  of  the  water,  contained  in  a 
loo  c.c.  Nessler  cylinder,  as  compared  with  a  similar  column 
of  distilled  water. 

Distilled  water  is  practically  colourless  when  examined 
in  this  way.  The  presence  of  organic  matter  may  confer 
a  pale  yellow  tint.  Moorland  water  containing  peaty  sub- 
stances is  often  decidedly  yellow  or  even  reddish-brown. 

Conductivity. — The  conductivity  of  ideally  pure  water, 
expressed  in  gemmhos,1  is  about  0-04  at  18°,  whereas  that  of 
ordinary  distilled  water  is  from  twenty-five  to  one  hundred 
times  greater,  say  about  3  gemmhos.  Natural  waters  have 
a  much  higher  conductivity  than  distilled  water,  and  the 
conductivity  of  a  natural  water  is  due  chiefly  to  the  saline 
impurities  dissolved  in  it 

A  portable  apparatus,  known  as  the  "Dionic  Water 
Tester,"  has  been  devised  for  measuring  the  conductivity 
of  water.  The  apparatus  consists  of  a  glass  cell  fitted  with 
platinum  electrodes,  and  a  small  continuous-current  dynamo2 
which  is  connected  with  the  electrodes  and  with  a  direct- 
reading  conductivity  meter.  The  dynamo  is  turned  by  hand, 
and  the  conductivity  of  the  water  in  the  cell  is  registered 

1  i  gemmho  =  i-ox  icr6  reciprocal  ohms. 

2  On  account  of  the  polarisation  of  the  electrodes  caused  by  a  direct 
current,   an  alternating   current  is  generally  used  for  measuring  the 
conductivity  of  solutions.      In   the  apparatus   described,   the   dynamo 
generates  a  direct  current  at  a  constant  pressure  of  100  volts,  and  the 
polarisation  electromotive  force,  which  amounts  to  about  2  volts,  is  taken 
into  account  when  adjusting  the  scale  on  the  conductivity  meter.     The 
error  introduced  by  any  variation  in  the  polarisation  electromotive  force 
is  negligibly  small. 


292  WATER  ANALYSIS 

on  a  dial  provided.  The  only  correction  is  for  temperature, 
and  the  whole  measurement  occupies  but  a  few  minutes. 

For  average  waters,  the  conductivity  at  20°  may  vary 
from  30  to  300  or  more.  The  conductivity  affords  no 
information  regarding  the  nature  of  the  dissolved  salts, 
but  for  a  very  dilute  solution  of  a  given  salt  at  constant 
temperature,  it  is  roughly  proportional  to  the  amount  of 
salt  present  If  a  water  contains  only  a  small  quantity  of 
sodium  salts,  an  approximate  estimate  of  the  amount  of 
calcium  and  magnesium  salts,  i.e.,  of  the  "hardness,"  may 
be  obtained.  At  20°,  every  20  units  of  conductivity 
correspond,  very  roughly,  to  one  "degree"  of  hardness. 
For  example,  a  certain  water  supply  was  found  to  have 
a  conductivity  of  540  at  20°,  which  would  indicate  a 
hardness  of  from  25°  to  30°.  The  hardness  was  found  to 
be  actually  28°. 

The  determination  of  conductivity  is  a  most  convenient 
and  rapid  method  of  periodically  testing  one  and  the  same 
water  supply.  Any  abnormal  variation  in  the  amount  of 
dissolved  salts — which  can  only  be  due  to  an  influx  of  water 
of  a  different  character  and,  it  may  be,  from  an  objectionable 
source — is  at  once  readily  detected. 


CHEMICAL   EXAMINATION. 

The  chemical  examination  includes  the  determination  of, 
more  especially,  the  dissolved  solids,  ammonia,  organic 
matter,  chloride,  nitrite,  nitrate,  "hardness,"  acidity  or 
alkalinity,  and  the  action  of  the  water  on  metals — especially 
lead.  For  some  purposes,  an  accurate  analysis  of  the  saline 
constituents  may  also  be  required. 

Total  Solids. 
(Residue  on  Evaporation^) 

The  residue  obtained  by  evaporating  a  sample  of  water 
to  dryness  consists  of  all  the  non-volatile  inorganic  and 
organic  substances  which  were  dissolved  or  suspended  in 
the  water.  If  the  water  is  very  turbid,  it  is  usually  desirable 
to  determine  the  solid  matter  in  suspension,  as  described  on 


TOTAL  SOLIDS  293 

p.  290.     The  filtered  water  is  then  used  for  the  determination 
of  the  dissolved  solids. 

The  quantity  of  water  which  should  be  evaporated 
depends  on  the  probable  amount  of  the  saline  constituents, 
and  from  100  c.c.  to  500  c.c.  is  usually  sufficient.  If  the  sample 
contains  10  parts  of  dissolved  solids  in  100,000,  the  residue 
from  250  c.c.  weighs  0025  gram. 

Procedure. — Place  a  clean  platinum  basin  in  an  air-oven 
at  1 80°  for  several  minutes,  then  cool  in  a  desiccator  for  ten 
minutes,  and  weigh. 

Measure  the  water  in  a  graduated  flask,  transfer  about 
50  c.c.  to  the  basin,  and  evaporate  on  the  steam-bath.  Add 
more  of  the  water  at  intervals,  and  finally  rinse  the  flask  with 
a  little  distilled  water  and  add  this  to  the  basia 

After  complete  evaporation,  remove  the  basin,  wipe  the 
outside  carefully,  and  place  it  in  an  air-oven  at  180°  for  an  hour. 
Cool  in  a  desiccator  for  ten  minutes,  and  weigh.  The  residue 
is  often  hygroscopic,  as  indicated  by  the  steadily  increasing 
weight  during  the  weighing.  Replace  the  basin,  therefore, 
in  the  air-oven  for  fifteen  minutes,  and  weigh  again  as 
rapidly  as  possible.  Repeat  until  constant  weight  is 
attained. 

The  results  are  not  comparable  unless  the  residue  is 
always  dried  at  the  same  temperature.  Dried  at  100°,  the 
residual  salts  retain  much  combined  water  (water  of  crystal- 
lisation). Calcium  sulphate  retains  half  a  molecule  at  107°, 
and  magnesium  sulphate  and  calcium  chloride  I  molecule 
even  at  180°. 

After  weighing,  and  in  order  to  obtain  an  idea  of  the 
amount  of  organic  matter  present  in  the  residue,  heat 
the  dish  gently  over  a  Bunsen  flame.  Observe  whether  the 
residue  changes  colour  (becoming  yellow,  brown,  or  black). 
A  smell  of  singed  horn  indicates  the  presence  of  nitrogenous 
organic  matter. 

The  residue  may  be  used  to  test  for  phosphate,  as 
described  on  p.  302. 

"Free"   Ammonia  and  " Albumenoid "  Ammonia. 

Ammonia  is  often  present  in  natural  water,  in  the  form 
of  ammonium  salts,  and  is  usually  referred  to  as  "  free "  or 


294 


WATER  ANALYSIS 


"  saline"  ammonia.  By  distilling  the  water  with  a  little 
sodium  carbonate,  the  ammonia  is  found  in  the  first  portion 
of  the  distillate. 

The  so-called  "  albumenoid "  ammonia  is  obtained  by 
distilling  the  water  with  alkaline  potassium  permanganate 
solution  after  the  removal  of  the  "free"  ammonia.  The 
organic  matter  present  in  the  water  is  decomposed  by  this 
treatment,  but,  as  only  a  portion  of  the  total  organic  nitrogen 
is  thereby  converted  into  ammonia,  the  results  afford  only 
an  approximate  measure  of  the  nitrogenous  organic  sub- 
stances present  in  the  water. 

The  ammonia  obtained  in  the  distillates  is  determined 
colorimetrically  by  means  of  Nessler  solution. 

The  following  materials  are  required  : — 

(i)  Nessler  Solution ;  (2)  Standard  Ammonium  Chloride 
Solution,  and  (3)  Ammonia-free  Water.  These  are  prepared 

as  described  on  p.  159. 

(4)  Alkaline    Potassium     Per- 
manganate Solution.  —  Dissolve   4 
grams  of  potassium  permanganate 
in  750  c.c.  of  water,  add  100  grams 
of  sodium  hydroxide,  and  boil  the 
solution  in  a  flask  until  the  volume 
is  reduced  to  about  500  c.c.     After 
the  solution  has  become  cold,  trans- 
fer it  to  a  stoppered  bottle. 

(5)  Ignited  Sodium   Carbonate. 
— Heat    some   sodium    carbonate 
crystals  in  a  platinum  crucible  to 
dull  redness  for  about  half  an  hour, 
then  allow  to  cool,  and  transfer  to 
a  stoppered  bottle. 

The   Apparatus  consists   of  a 
distilling    flask   of  not   less   than 
i  litre  capacity,  and  a  condenser. 
The  distilling  flask,  if  not   fitted 
IG*  74'  with  a  ground-in  stopper,  is  closed 

with  a  tube  on  which  a  bulb  is  blown  and  which  contains 
lead  shot  in  order  to  give  it  more  weight ;  a  piece  of  clean 
sheet  rubber,  in  which  a  small  hole  is  cut  with  a  cork  borer, 


FREE  AMMONIA  295 

is  drawn  over  this  tube  below  the  bulb,  and  the  bulb  rests 
on  and  closes  the  mouth  of  the  flask.  The  side-tube  of  the 
distilling  flask  should  pass  well  down  into  the  inner  tube  of 
the  condenser,  and,  if  the  tubes  fit  closely,  no  connecting 
cork  need  be  used.  If  a  rubber  cork  is  used  it  should  be 
soaked  in  sodium  hydroxide  solution  and  then  thoroughly 
washed. 

The  determination  of  ammonia  must  be  conducted  in  a 
room  which  contains  no  ammonia  or  ammonium  salts,  and 
the  apparatus  must  not  be  used  for  any  other  purpose. 

Preliminary  Examination  of  the  Water. — Measure  50 
c.c.  of  the  sample  into  a  Nessler  cylinder,  add  2  c.c.  of 
Nessler  solution,  and  mix.  From  a  burette,  run  0-5  c.c.  of 
the  standard  ammonium  chloride  solution  into  another 
cylinder,  fill  to  the  mark  with  ammonia-free  water,  add  2  c.c. 
of  Nessler  solution,  and  mix.  If  the  colorations  are  of  about 
equal  intensity,  use  500  c.c.  of  the  water  for  the  determination. 
As  a  rule,  half  a  litre  of  a  potable  water  is  a  suitable  quantity. 

Determination  of  "Free"  Ammonia. — Rinse  the  flask 
and  condenser  with  hydrochloric  acid,  and  then  wash 
thoroughly  with  water.  Place  about  200  c.c.  of  distilled 
water  and  about  I  gram  of  recently  ignited  sodium  carbonate 
in  the  flask,  and  distil  fairly  rapidly  with  a  free  flame.  Allow 
the  steam  to  blow  through  the  condenser  for  several  minutes, 
then  admit  the  condensing  water,  and  collect  the  distillate  in 
Nessler  cylinders.  Continue  the  distillation  until  the  distillate 
gives  no  coloration  with  Nessler  solution  after  standing  for 
three  minutes. 

When  the  apparatus  has  been  proved  in  this  way  to  be 
ammonia-free,  remove  the  flame  and  introduce,  through  a 
long-stemmed  funnel,  500  c.c.  of  the  water  (or  other  suitable 
quantity,  as  determined  by  the  preliminary  examination), 
and  proceed  with  the  distillation.  Distil  fairly  rapidly  and 
as  regularly  as  possible,  and  collect  the  distillate  in  three 
portions  of  50  c.c.  each,  using  Nessler  cylinders  as  receivers. 
Then  interrupt  the  distillation  and  allow  the  apparatus  to 
remain  for  the  determination  of  the  "albumenoid  "  ammonia 
in  the  residue.  The  whole  of  the  ammonia  in  the  portion  of 
water  taken  is  now  contained  in  the  distillate,  and  is  deter- 
mined colorimetrically  as  described  on  p.  159. 


296  WATER  ANALYSIS 

Determine  the  ammonia  in  the  second  portion  of  the 
distillate  before  the  first  portion  is  examined,  in  case  the 
latter  contains  too  much  ammonia  (more  than  01  mgrm.)  for 
accurate  comparison.  If  the  second  portion  requires  not 
more  than  2  c.c.  of  the  standard  ammonium  chloride  solution 
to  give  a  colour  of  equal  intensity,  the  whole  of  the  first 
portion  should  be  examined  in  the  same  way.  If  the  second 
portion  requires  more  than  2  c.c.,  take  one-half  of  the  first 
portion  (after  mixing  it  well)  and  dilute  to  50  c.c.  with 
ammonia-free  water  before  adding  the  Nessler  solution ;  the 
total  ammonia  in  the  first  portion  is  then  twice  the  observed 
quantity.  Then  test  the  third  portion  ;  it  will  probably 
contain  no  ammonia. 

From  the  total  volume  of  the  standard  ammonium 
chloride  solution  required  for  the  whole  of  the  distillate, 
calculate  the  weight  of  ammonia  in  100,000  parts  of  the  water. 
One  c.c.  of  the  standard  solution  corresponds  to  o-oi  mgrm. 
of  NH3,  and  if  4  c.c.  in  all  were  required,  and  500  c.c.  of 
water  used,  the  water  contained  0-008  part  of  ammonia  in 
100,000. 

Determination  of  "  Albumenoid "  Ammonia. — Add  to 
the  contents  of  the  distillation  flask  50  c.c.  (one-tenth  of  the 
volume  of  water  used)  of  the  alkaline  permanganate  solution, 
and  continue  the  distillation  until  four  portions  of  50  c.c. 
have  been  collected.  Test  each  portion  of  the  distillate, 
beginning  with  the  last  one. 

Calculate  the  amount  of  albumenoid  ammonia  which  is 
obtained  from  100,000  parts  of  the  water. 

Reducing  Power. 

(Organic  Matter) 

The  organic  matter  in  natural  waters  is  mainly  of  plant 
or  animal  origin,  and  consists  essentially  of  the  decomposi- 
tion products  of  decayed  vegetation  and  of  substances 
derived  from  the  animal  organism.  Many  of  these  organic 
substances — more  particularly  those  derived  from  animal 
sources — contain  nitrogen,  and  one  of  the  simplest  methods 
of  obtaining  information  regarding  the  amount  of  organic 
matter  in  water  is  the  determination  of  the  "  a.lbqmenoi4  " 
Ammonia,  already  desgribec). 


ORGANIC  MATTER  297 

The  organic  matter  is  more  or  less  easily  oxidised,  and 
an  approximate  estimate  of  the  organic  purity  of  a  water 
may  also  be  arrived  at  by  determining  its  reducing  power, 
or  its  power  of  combining  with  or  "absorbing"  oxygen. 
The  reducing  power  of  a  water  is  usually  ascertained  by 
measuring  its  action  on  a  dilute  acid  solution  of  potassium 
permanganate.  The  following  solutions  are  required  : — 

(1)  Potassium  Permanganate  Solution  (N/8o). — Dissolve 
0-395  gram  of  the   pure  salt  in  cold  water,  and  dilute  the 
solution  to  I  litre.      (One  c.c.  corresponds  to  o-i   mgrm.  of 
available  oxygen.) 

(2)  Dilute  Sulphuric  Acid. — Pour   100  c.c.  of  pure  con- 
centrated  sulphuric   acid   into   300  c.c.  of  water,   cool   the 
mixture  partially,  and  add    dilute  potassium  permanganate 
solution,  drop  by  drop,  until  the  acid  acquires  a  permanent 
pink  tinge. 

(3)  Sodium   Thiosulphate   Solution   (N/25O). — Dissolve    I 
gram  of  the  pure  salt  in  I  litre  of  water. 

(4)  Potassium  Iodide  Solution. — Dissolve  10  grams  in  100 
c.c.  of  water. 

Procedure. — Measure  250  c.c.  of  the  water  to  be  examined 
into  a  perfectly  clean  stoppered  bottle  (500  c.c.).  Place  the 
bottle  in  a  thermostat  or  air-bath  at  27°,  and,  when  the 
temperature  of  the  water  has  become  constant,  add  10  c.c.  of 
the  dilute  sulphuric  acid  and  loc.c.  of  the  potassium  perman- 
ganate solution.  Allow  the  bottle  to  remain  in  the  bath  for 
four  hours.1  Examine  it  from  time  to  time,  and  if  a  marked 
diminution  in  the  depth  of  the  pink  colour  is  observed,  add 
another  10  c.c.  of  the  permanganate  solution. 

Meanwhile  standardise  the  sodium  thiosulphate  solution 
as  follows : — Place  250  c.c.  of  freshly  distilled  water  in  a 
500  c.c.  bottle,  add  10  c.c.  of  the  sulphuric  acid,  10  c.c.  of 
the  permanganate  solution,  and  then  I  c.c.  of  the  potassium 
iodide  solution.  Titrate  the  liberated  iodine  with  the 
thiosulphate,  starch  solution  being  used  as  indicator  as  usual. 
The  volume  of  thiosulphate  solution  required  corresponds 
to  i  mgrm.  of  available  oxygen. 

1  Widely  varying  temperatures  and  periods  of  digestion  are  employed, 
and  the  results  are  not  comparable  unless  obtained  by  identical  methods. 


298  WATER  ANALYSIS 

After  the  lapse  of  four  hours,  remove  the  bottle  contain- 
ing the  water  sample  and  cool  the  contents  by  immersing 
the  bottle  in  water.  Add  I  c.c.  of  the  potassium  iodide 
solution,  and  titrate  with  the  thiosulphate. 

If  30  c.c.  of  the  thiosulphate  corresponds  to  I  mgrm.  of 
oxygen,  and  if  24  c.c.  were  required  after  digesting  the  water 
with  the  permanganate,  the  amount  of  oxygen  absorbed  by 

250  c.c.  of  the  water  is =  0-2  mgrm.,  i.e.,  0-08  part 

per  100,000. 

Note. — If  the  water  contains  appreciable  amounts  of 
ferrous  salts,  nitrite,  or  sulphide,  an  immediate  reduction  of 
a  corresponding  amount  of  the  permanganate  occurs,  and  a 
separate  experiment  must  be  made  in  order  to  determine 
how  much  of  the  oxygen  absorbed  is  due  to  the  inorganic 
substances  present.  In  this  case,  the  mixture  containing 
the  water,  sulphuric  acid,  and  potassium  permanganate  is 
allowed  to  remain  in  the  thermostat  for  only  five  minutes, 
after  which  it  is  rapidly  cooled,  potassium  iodide  is  added, 
and  the  iodine  titrated  as  before. 

This  correction  is  seldom  necessary  in  the  case  of 
potable  waters. 

Chloride. 

All  natural  water  contains  chloride.  Rain  water  may 
contain  only  a  mere  trace,  and  river  water  less  than  i 
part  of  "chlorine"  in  100,000,  but  a  much  larger  quantity  is 
often  present,  especially  in  well  water. 

The  following  solutions  are  required: — (i)  A  solution  of 
silver  nitrate  containing  4-79  grams  per  litre.  One  c.c.  of 
this  solution  corresponds  to  i  mgrm.  of  chlorine.  (2)  A 
2  per  cent,  solution  of  potassium  chromate,  free  from  chloride 
(see  p.  99). 

Procedure. — Measure  100  c.c.  of  the  water  into  a  porce- 
lain basin,  add  i  c.c.  of  the  potassium  chromate  solution, 
and  run  in  the  silver  nitrate  solution  from  a  burette,  whilst 
stirring  briskly,  until  a  faint  permanent  reddish  tinge  is 
obtained.  On  account  of  the  solubility  of  silver  chromate, 
a  slight  excess — about  0-2  c.c. — of  silver  nitrate  must  be 


CHLORIDE—NITRITE  299 

added  before  the  reddish  tinge  (precipitated  silver  chromate) 
is  apparent.  The  number  of  cubic  centimetres  of  silver 
nitrate  solution  used,  less  0-2  c.c.,  gives  the  milligrams 
of  chloride  (Cl)  in  100  c.c.  of  the  water,  i.e.,  parts  per 
100,000. 

Alternative  Procedure. — To  100  c.c.  of  the  water,  add 
i  c.c.  of  the  potassium  chromate  solution.  Run  in  the  silver 
nitrate  solution  until,  after  stirring,  a  permanent  reddish 
tinge  is  obtained.  Then  add,  from  a  graduated  cylinder, 
more  of  the  water  sample  until  the  reddish  tinge  is  just 
destroyed.  (From  5  to  20  c.c.  may  be  required.)  The 
volume  of  silver  nitrate  solution  used  corresponds  to  the 
chloride  in  100  c.c.  of  the  water  plus  one-half  of  the  further 
quantity  of  water  added.  Calculate  the  number  of  cubic 
centimetres  of  silver  nitrate  solution  required  for  100  c.c.  of 
the  water. 

Notes. — If  the  water  contains  less  than  i  part  of 
chloride  in  100,000,  use  a  larger  volume — say  250  c.c. — and 
evaporate  on  the  steam-bath  to  about  100  c.c.,  then  cool,  and 
titrate. 

If  the  water  is  acid,  add  a  little  pure  sodium  bicarbon- 
ate solution  (sufficient  to  render  the  water  faintly  alkaline) 
before  titrating. 

If  the  water  is  peaty  and  highly  coloured,  or  if  it  is 
sewage-polluted  and  contains  hydrogen  sulphide,  add  a 
small  quantity  of  lime-water  (free  from  chloride)  to  the 
measured  portion  of  the  water;  pass  a  current  of  carbon 
dioxide  through  the  liquid,  then  boil  for  a  few  minutes, 
cool,  and  filter.  Determine  the  chloride  in  the  filtrate  as 
directed. 

Nitrite. 

Nitrite  is  rarely  found  in  natural  waters,  except  in  traces, 
and  an  accurate  quantitative  determination  is  seldom 
necessary. 

Detection  of  Nitrite. — To  50  c.c.  of  the  water,  add  I  c.c. 
of  freshly  prepared  starch  solution,  a  few  drops  of  dilute 
sulphuric  acid,  and  then  i  c.c.  of  potassium  iodide  solution. 
Nitrite  is  present  if  a  blue  colour  develops  within  two 
minutes.  The  colour  should  appear  in  half  a  minute  if 


300  WATER  ANALYSIS 

the  water  contains  about  0-03  part  of  nitrite  (NO2)  in 
100,000. 

A  parallel  test  should  be  made  with  distilled  water  in 
order  to  make  certain  that,  with  the  reagents  employed,  no 
coloration  is  produced  in  the  absence  of  nitrite. 

Quantitative  Determination. — Prepare  a  standard  nitrite 
solution  as  follows: — Dissolve  0-335  gram  of  pure  silver 
nitrite  in  hot  water,  add  excess  of  sodium  chloride  (about 
0-2  gram),  cool,  and  dilute  to  I  litre.  (Keep  this  solution 
in  the  dark.)  For  immediate  use,  dilute  5  c.c.  of  the 
clear  liquid  with  tap-water  (free  from  nitrite),  and  dilute  to 
i  litre.  This  solution  contains  0-05  part  of  nitrite  (NO2)  in 
100,000. 

Place  50  c.c.  of  the  water  sample  and  of  the  standard 
nitrite  solution  into  two  Nessler  cylinders ;  add  I  c.c.  of 
starch  solution  and  I  c.c.  of  dilute  sulphuric  acid  to  each ; 
and  then  add,  as  nearly  as  possible  at  the  same  moment, 
I  c.c.  of  potassium  iodide  to  each.  If  the  blue  colour  appears 
sooner  in  the  water  sample  than  in  the  standard,  use  a 
smaller  quantity  of  the  sample,  and  dilute  with  tap-water 
(free  from  nitrite)  to  50  c.c. ;  if  it  develops  more  rapidly 
in  the  standard,  dilute  the  latter ;  and,  in  either  case,  repeat 
the  experiment  until  it  is  found  that  the  colour  appears  simul- 
taneously in  both  tubes. 

The  amount  of  nitrite  (NO2)  in  the  water  can  then  be 
calculated. 

Nitrate. 

Practically  all  natural  waters  contain  at  least  a  trace 
of  nitrate.  Nitrate,  like  chloride,  is  itself  innocuous,  but, 
from  a  hygienic  standpoint,  the  presence  of  a  large 
amount  of  nitrate  in  a  drinking  water  is  by  no  means 
unimportant. 

Detection  of  Nitrate. — In  a  test-tube,  mix  i  c.c.  of  the 
water,  drop  by  drop,  with  3  c.c.  of  concentrated  sulphuric 
acid,  cool,  and  then  add  a  few  milligrams  of  brucine. 

If  nitrate  is  present,  a  red  coloration  is  obtained.  With 
10  parts  of  nitrate  (NO3)  in  100,000,  the  colour  is  cherry-red, 
changing  to  orange ;  with  i  part,  it  is  rose-red  ;  and  with 
o-i  part,  a  pale  rose-coloured  solution  is  obtained. 


NITRATE  301 

In  case  the  sulphuric  acid  contains  a  trace  of  nitrate,  it  is 
essential  to  make  a  parallel  experiment  with  distilled  water, 
and,  if  nitrate  is  present,  to  compare  the  coloration  with  that 
given  by  the  water  sample.  Nitrite  gives  no  coloration  with 
brucine  and  sulphuric  acid,  provided  the  above  directions  are 
exactly  followed.  Ferrous  salts  interfere  and,  if  present, 
must  therefore  be  removed  by  means  of  sodium  hydroxide 
free  from  nitrate. 

Quantitative  Determination. — The  nitrate  is  reduced  to 
an  ammonium  salt  by  means  of  a  copper-zinc  couple,  and 
the  ammonia  is  then  determined  by  means  of  Nessler  solu- 
tion. From  10  c.c.  to  100  c.c.  of  the  water,  according  to  the 
amount  of  nitrate  present,  is  required  for  the  determination. 
If  about  i  part  of  nitrate  (NO3)  in  100,000  is  probably 
present,  use  50  c.c. 

Preparation  of  the  Copper-Zinc  Couple. — Roll  a  piece  of 
zinc  foil,  3  inches  square,  into  a  cylinder  about  f  inch 
diameter.  Dip  the  foil  into  hydrochloric  acid  in  order  to 
clean  it,  and  then  wash  it  with  water.  Place  it  in  a  large 
test-tube,  and  cover  it  with  a  3  per  cent,  solution  of  copper 
sulphate  for  a  few  minutes  until  it  is  completely  coated  with 
a  black  deposit  of  copper.  Pour  away  the  copper  sulphate 
solution,  and  wash  the  foil  with  distilled  water  without 
removing  it  from  the  test-tube. 

Reduction  of  the  Nitrate. — If  the  water  contains  much 
ammonia,  dilute  the  measured  portion  (if  necessary)  to  about 
100  c.c.,  and  boil  it  until  the  volume  is  reduced  to  about  50 
c.c.  Cool  the  water,  transfer  it  to  the  test-tube  containing  - 
the  copper-zinc  couple,  and  add  dilute  hydrochloric  acid 
(3  or  4  drops)  sufficient  to  cause,  after  several  minutes,  a 
slight  evolution  of  hydrogen.  Cover  the  test-tube  with  a 
watch-glass,  and  set  aside  overnight. 

The  reduction  is  more  rapid  at  higher  temperatures,  and 
at  50°  it  is  complete  in  an  hour. 

Determination  of  the  Ammonia. — Place  about  400  c.c.  of 
distilled  water  and  about  I  gram  of  ignited  sodium  carbonate 
in  an  apparatus  similar  to  that  used  for  the  determination 
of  ammonia  in  water,  and  remove  any  ammonia  in  the 
apparatus  by  distilling  over  about  100  c.c.  Transfer  the 
contents  of  the  test-tube  to  the  distilling  flask,  and  add 


302  WATER  ANALYSIS 

the  washings  of  the  copper-zinc  couple.  Distil  off  the 
ammonia  (in  two  portions  of  50  c.c.),  and  determine  it 
colorimetrically  as  described  on  p.  159. 

From  the  amount  of  ammonia  thus  found,  calculate  the 
corresponding  amount  of  nitrate  (NO3).  Any  nitrite  in 
the  water  is  of  course  included,  but  the  amount  is  usually 
so  small  as  not  to  appreciably  affect  the  result. 

Phosphate. 

Phosphate  is  rarely  present  in  natural  waters,  except 
in  traces.  As  a  rule,  only  a  qualitative  test  need  be 
made. 

Dissolve  the  residue  obtained  in  the  determination  of  the 
"  Total  Solids "  (p.  293)  in  dilute  .nitric  acid,  transfer  the 
solution  to  a  porcelain  basin,  and  evaporate  to  dryness  on 
the  steam-bath.  Moisten  the  residue  with  nitric  acid,  and 
again  evaporate  to  complete  dryness.  Then  add  a  few  drops 
of  nitric  acid  and  5  c.c.  of  water,  warm,  and  filter.  Add  5  c.c. 
of  ammonium  molybdate  solution,  warm  to  about  60°,  and 
set  aside.  If  phosphate  is  present,  a  yellow  turbidity,  or  a 
distinct  precipitate,  is  obtained,  and  its  amount  may  be 
approximately  estimated  by  comparing  it  with  known 
amounts  of  ammonium  phospho-molybdate. 

Hardness. 

Natural  waters  are  familiarly  described  as  "  hard "  or 
"  soft,"  these  terms  having  reference  to  the  action  of  the 
water  on  soap.  A  water  is  said  to  be  "  soft "  if  it  gives  an 
immediate  lather  with  soap,  and  "hard"  if  a  lather  is 
obtained  with  difficult)'.  The  "  hardness "  of  a  water  thus 
refers  to  its  soap-destroying  power,  and  is  due  almost 
entirely  to  the  calcium  and  magnesium  salts  dissolved  in 
the  water. 

Hardness  is  expressed  in  degrees,  i°  corresponding  to 
I  part  of  calcium  carbonate,  or  its  equivalent  in  other  calcium 
and  magnesium  salts,  in  100,000  parts  of  water. 

It  is  usual  to  distinguish  between  u temporary"  and 
"  permanent "  hardness.  The  former  is  due  to  the  presence 


TEMPORARY  HARDNESS  303 

of   calcium    and    magnesium   bicarbonates,   and   the   latter 
mainly  to  the  corresponding  sulphates  and  chlorides. 

Temporary  Hardness  is,  strictly  speaking,  the  hardness 
that  disappears  on  boiling  the  water.  The  bicarbonates  are 
thereby  decomposed  and  normal  carbonates  are  precipitated  ; 
but,  as  the  solubility  of  normal  calcium  carbonate  in  boiling 
water  amounts  to  about  2  parts  in  100,000,  and  as  magnesium 
carbonate  is  still  more  soluble,  no  diminution  in  hardness 
will  occur  on  boiling  unless  the  original  hardness,  due  to 
bicarbonate,  exceeds  at  least  2°. 

Permanent  Hardness,  i.e.,  the  hardness  that  remains  after 
boiling  the  water,  is  thus  due  not  only  to  the  sulphates  and 
chlorides  of  calcium  and  magnesium,  but  also  to  the  calcium 
and  magnesium  carbonates  that  still  remain  dissolved. 

The  hardness  of  a  water,  or  its  soap-destroying  power, 
may  be  determined  by  means  of  a  standardised  solution  of 
soap.  The  soap  solution  is  gradually  added  to  a  measured 
volume  of  the  water  until,  after  vigorous  agitation,  a 
permanent  lather  is  obtained.  The  hardness  determined  in 
this  way  corresponds  approximately  to  the  amount  of  calcium 
and  magnesium  salts  dissolved  in  the  water.  When  it 
is  necessary  to  ascertain  the  respective  amounts  of  these 
salts  accurately,  the  calcium  and  magnesium  must  be  deter- 
mined gravimetrically  as  described  on  p.  314,  but  for  many 
practical  purposes  sufficient  information  is  obtained 
more  expeditiously  by  the  volumetric  methods  described 
below. 

Determination  of  Temporary  Hardness  or  "Alka- 
linity."— The  bicarbonates  of  calcium  and  magnesium  are, 
as  a  rule,  the  only  substances  in  a  natural  water  which,  on 
account  of  hydrolysis,  confer  alkalinity  (as  shown  by  certain 
indicators)  on  the  water,  and  the  bicarbonate  hardness  is 
sometimes  referred  to  as  the  "  alkalinity "  of  the  water. 
The  "  alkalinity  "  is  determined  as  follows  : — 

Measure  100  c.c.  of  the  water  into  a  porcelain  basin,  and 
heat  until  boiling.  Add  2  c.c.  of  methyl  red,  and  run  in 
0-02  N  hydrochloric  acid  until  the  solution  begins  to  appear 
permanently  reddened.  Boil  again  for  about  a  minute;  if 
too  much  acid  has  not  been  added,  the  yellow  colour  of  the 
indicator  will  reappear.  Add  more  acid,  drop  by  drop,  until 


304  WATER  ANALYSIS 

the  first  sign  of  a  red  tinge,  which  is  not  destroyed  by  further 
boiling,  is  obtained.  The  end-point  of  the  titration  is  sharply 
defined ;  but,  in  case  of  any  difficulty,  it  may  be  found  easier 
to  add  the  standard  acid  until  the  solution  is  decidedly  red, 
and  then,  after  boiling,  to  titrate  the  excess  of  acid  with 
0-02  N  baryta  solution. 

One  c.c.  of  0-02  N  acid  corresponds  to  o-ooi  gram  of 
CaCO3;  and,  since  100  c.c.  of  the  water  was  taken,  each 
cubic  centimetre  of  the  standard  acid  used  in  the  titration 
represents  I  part  of  calcium  carbonate  in  100,000  parts  of 
water,  or  i°  of  hardness. 

Some  natural  waters  contain  sodium  carbonate,  or  may 
have  been  "  softened  "  by  the  addition  of  sodium  carbonate, 
and  the  "alkalinity"  may  be  due  partly  to  the  latter.  In 
that  case,  a  correction,  referred  to  in  the  next  paragraph, 
must  be  applied.  The  "alkalinity"  is  then  equal  to  the 
temporary  hardness  or,  more  correctly,  to  the  bicarbonate 
hardness. 

Determination  of  Permanent  Hardness.  —  Measure 
loo  c.c.  of  the  water  into  a  platinum  or  a  silica  basin,  add 
10  c.c.1  of  approximately  0-04  N  sodium  carbonate,  and 
evaporate  to  dryness  on  the  steam-bath.2  Extract  the  residue 
repeatedly  with  30  per  cent,  alcohol  (30  c.c.  of  pure,  absolute 
alcohol — not  rectified  spirit — diluted  to  100  c.c.,  and  contained 
in  a  small  wash-bottle)  as  follows : — Rinse  the  sides  of  the 
basin  with  the  alcohol  and,  after  a  few  minutes,  filter  through 
a  5j  cm.  ("black  ribbon")  filter  paper  into  a  porcelain  basin. 
Repeat  the  extraction  process  five  times,  and  then  wash  the 
filter  six  times  with  the  dilute  alcohol.  (Set  aside  the 
platinum  basin  and  the  filter  for  the  determination  of  total 
hardness,  as  described  below.) 

Add  I  c.c.  of  methyl  red  to  the  filtrate  and  titrate  at  the 
boiling-point  with  0-02  N  hydrochloric  acid  (or  add  excess  of 
the  standard  acid,  boil  for  a  minute,  and  titrate  the  excess 

1  If  the   permanent  hardness  is  believed   to  exceed  15°,  more  than 
10  c.c.  of  the  sodium  carbonate  must  be  added. 

2  The  evaporation  must  be  conducted  in  such  a  manner  as  to  avoid 
contamination  of  the  contents  of  the  basin  with  oxides  of  sulphur  from  a 
coal-gas  flame.     A  steam-bath  with  an  external  source  of  steam  (see 
Fig.  10,  p.  22)  is  probably  essential  for  satisfactory  results. 


TOTAL  HARDNESS  305 

with  002  N  baryta  solution),  as  described  under  temporary 
hardness.  In  the  same  manner,  titrate  10  c.c.  of  the 
original  sodium  carbonate  solution,  previously  diluted  to 
about  50  c.c. 

The  permanent  hardness,  in  degrees,  is  equal  to  the 
amount  of  sodium  carbonate,  measured  in  cubic  centimetres 
of  the  002  N  acid,  required  to  precipitate  the  calcium  and 
magnesium  sulphates  and  chlorides  in  100  c.c.  of  the  water. 
If,  for  example,  10  c.c.  of  the  sodium  carbonate  solution 
required  20-45  c-c-  °f  °'°2  N  acid,  and  the  residual  sodium 
carbonate  in  the  filtrate  required  16-30  c.c.,  the  permanent 
hardness  is  equal  to  20-45  —  16-30  =  4-15°. 

If  the  original  water  contains  sodium  carbonate,  there  can 
be  no  permanent  hardness,  and,  in  the  foregoing  determina- 
tion, a  gain  instead  of  a  loss  of  sodium  carbonate  will  be 
found,  ?>.,  the  filtrate  will  require  more  acid  than  that 
necessary  to  neutralise  the  sodium  carbonate  added  to  the 
water.  The  excess  represents  the  amount  of  sodium 
carbonate  originally  present  in  the  water,  and  it  must  be 
deducted  from  the  "alkalinity"  in  order  to  find  the  real 
bicarbonate  hardness. 

Determination  of  Total  Hardness.— The  sum  of  the 
temporary  and  the  permanent  hardness  is  the  total  hard- 
ness of  the  water.  The  total  hardness  may  also  be  deter- 
mined directly  in  the  following  manner  : — 

Measure  5  c.c.1  of  o-i  N  hydrochloric  acid  into  the  basin 
in  which  the  evaporation  described  under  permanent  hardness 
was  conducted.  (The  basin  and  filter  contain  the  precipitated 
carbonates  of  calcium  and  magnesium.)  Place  the  basin  on 
a  perforated  silica  plate,  and  warm  with  a  small  flame  until 
the  acid  just  boils.  Pour  the  acid  through  the  filter,  care 
being  taken  that  the  acid  comes  into  contact  with  every 
part  of  the  paper,  and  receive  the  filtrate  in  a  porcelain 
basin.  Rinse  the  basin  and  wash  the  filter  thoroughly 
with  hot  water.  Boil  the  filtrate  for  a  minute  on  a  silica 
plate,  add  I  c.c.  of  methyl  red,  and  titrate  with  0-02  N  baryta 
solution. 

The  total  hardness  of  the  water,  in  degrees,  is  equal  to 

1  Five  c.c.  is  sufficient  if  the  total  hardness  is  less  than  20°. 

U 


306  WATER  ANALYSIS 

the  number  of  cubic  centimetres  of  0-02  N  acid  required  to 
decompose  the  carbonates  in  100  c.c.  of  the  water. 

Determination  of  Calcium  and  Magnesium. — The 
calcium  is  precipitated  as  oxalate,  and  is  determined  volu- 
metrically  by  titration  with  standard  potassium  permanganate 
solution.  The  difference  between  the  total  hardness  and 
that  due  to  calcium  salts  gives  the  hardness  due  to  magnesium 
salts. 

Measure  100  c.c.  of  the  water  into  a  glass  basin,  add  I  c.c. 
of  dilute  hydrochloric  acid,  and  heat  until  almost  boiling.  To 
the  hot  solution,  add  ammonia  until  it  is  ammoniacal  and 
then  3  c.c.  of  a  freshly  prepared  saturated  solution  of 
ammonium  oxalate. 

Evaporate  on  the  steam-bath  until  the  volume  of  the 
liquid  is  about  10  c.c.  Add  10  c.c.  of  ammonia,  filter  the 
calcium  oxalate  through  a  small  filter  paper,  and  wash  it 
with  warm  water  containing  a  little  ammonia  until  the 
filtrate  is  free  from  chloride.  Heat  5  c.c.  of  dilute  sulphuric 
acid  in  the  original  glass  basin  (in  case  any  of  the  pre- 
cipitate remains  in  it),  pour  the  hot  acid  into  the  filter — 
care  being  taken  that  the  acid  comes  into  contact  with 
every  part  of  the  paper — and  receive  the  solution  in  a 
small  conical  flask.  Rinse  the  basin  and  wash  the  filter 
with  hot  water.  Titrate  the  hot  solution  with  0-02  N 
potassium  permanganate. 

Calculate  the  amount  of  calcium,  expressing  it  all  as 
carbonate,  in  100,000  parts  of  the  water. 

If  the  hardness  due  to  calcium  is  found  to  be  6°  and  the 
total  hardness  was  10°,  the  hardness  due  to  magnesium 
corresponds  to  4°.  But  since  i  part  of  calcium  carbonate 
corresponds  to  0-84  part  of  magnesium  carbonate,  the 
actual  amount  of  magnesium  (expressing  it  all  as  carbonate) 
is  4x0-84  =  3-36  parts  per  100,000. 

Note. — The  permanganate  solution  should  be  standardised 
by  means  of  a  dilute  solution  of  calcium  chloride  of  known 
concentration  (prepared  from  calcite),  under  conditions  similar 
to  those  described  above. 


RELATIVE  ACIDITY  AND  ALKALINITY  307 

Relative  Acidity  and  Alkalinity. 

Ideally  pure  water  is  neutral,  but  natural  waters  may  be 
neutral,  acid,  or  alkaline,  according  to  the  substances  dissolved 
in  them.  The  acidity  or  alkalinity  of  water  is  usually 
determined  by  means  of  an  indicator,  such  as  litmus  or 
phenolphthalein,  but  since  different  indicators  give  different 
results,  one  and  the  same  water  may  appear  acid  when 
tested  with  one  indicator,  and  alkaline  with  another.  Ordinary 
distilled  water,  for  example,  appears  acid  with  litmus  or 
phenolphthalein,  and  approximately  neutral  with  methyl 
orange. 

The  so-called  "  alkalinity  "  of  a  water,  as  determined  by 
titration  (compare  temporary  hardness),  is  really  a  measure 
of  the  potential  alkalinity  of  the  water,  and  not  of  its  actual 
alkalinity.  The  potential  alkalinity  of  a  solution  of  sodium 
bicarbonate,  for  example,  as  determined  by  titration  with 
methyl  orange  as  indicator,  is  the  same  as  that  of  a  solution 
of  sodium  hydroxide  of  the  same  molecular  concentration ; 
whereas  sodium  bicarbonate  solution  appears  neutral,  and 
sodium  hydroxide  solution  alkaline,  when  tested  with 
phenolphthalein.  The  actual  alkalinities  of  these  solutions 
are,  in  fact,  very  different,  and  it  is  on  the  actual  acidity  (or 
alkalinity)  that  many  of  the  properties  of  a  solution,  such  as 
the  rate  of  its  action  on  metals,  depends. 

Ideally  pure  water  and  all  neutral  solutions  contain  the 
acid  ion(hydrion)  H',  and  the  alkaline  ion  (hydroxidion)  OH', 
in  chemically  equivalent  proportions ;  and  the  product  of  the 
concentrations  of  these  ions  in  all  dilute  aqueous  solutions 
as  well  as  in  pure  water  is  constant.1  If  an  excess  of  either 
ion  is  present,  as  is  usually  the  case  in  ordinary  water,  the 
water  is  either  acid  or  alkaline,  and  the  acidity  or  alkalinity 
may  be  stated  in  terms  of  the  hydrion  (or  hydroxidion) 
concentration. 

It  is  more  convenient,  for  practical  purposes,  to  express 

1  The  exact  values  depend  on  temperature.  At  18°,  the  concentrations 
of  hydrion  and  hydroxidion  in  pure  water  are  each  equal  to  o'68x  io~7 
normal,  and  at  the  same  temperature  the  product  of  the  concentrations 
of  these  ions  in  water  and  in  any  dilute  aqueous  solution  is  therefore 
0-46  x  10  ~14. 


308  WATER  ANALYSIS 

the  acidity  or  alkalinity  of  ordinary  water  in  terms  of  the 
acidity  or  alkalinity  of  ideally  pure  water  at  the  same 
temperature.  If  the  acidity  of  pure  water  is  taken  as  I, 
then  the  alkalinity  is  also  I  ;  and  if  the  hydrion  con- 
centration in  a  sample  of  ordinary  water  is  twice  the 
hydrion  concentration  in  pure  water  at  the  same  tem- 
perature, it  would  be  stated  to  have  a  relative  acidity  of  2. 
(Its  relative  alkalinity  would  be  half  that  of  pure  water, 
viz.,  0-5.) 

The  following  method  of  determining  the  relative  acidity 
or  alkalinity  of  a  water  is  a  colorimetric  one,  and  is  based 
on  the  use  of  standard  solutions  of  definite  acidity  and 
alkalinity,  in  which  azolitmin  assumes  a  red  tint  when  the 
solution  is  decidedly  acid,  passes  through  intermediate 
shades  of  purple,  and  becomes  finally  blue  when  the  solu- 
tion is  decidedly  alkaline.  The  following  solutions  are 
required : — 

(1)  Disodium    Phosphate.  —  Dissolve    23-88     grams     of 
Na2HPO4,  I2H2O  in  water,  and  dilute  the  solution  to  I  litre. 
"Chemically    pure"   crystals,   free    from    efflorescence,    are 
satisfactory.     This  solution  is  alkaline. 

(2)  Potassium  Dihydrogen  Phosphate. — Dissolve  9-08  grams 
of  KH2PO4  in  water,  and  dilute  the  solution  to  i  litre.     The 
salt  must  be  free  from  chloride  and  sulphate.     This  solution 
is  acid. 

(3)  A   Neutral  Solution. — Mix  250  c.c.  of  the  disodium 
phosphate  solution  with  158  c.c.  of  the  potassium  dihydrogen 
phosphate  solution.     The  solutions  may  be  measured  with 
sufficient  accuracy  in  a  narrow  graduated  cylinder. 

(4)  Azolitmin. — Dissolve  o-i  gram  of  azolitmin  in  100  c.c. 
of  water. 

Two  methods  of  procedure  are  described  below.  The 
first  method  is  suitable  for  waters  which  are  practically 
free  from  colour,  and  the  second  method  is  used  for 
coloured  waters.  If  the  water  is  turbid,  the  suspended 
matter  must  be  allowed  to  subside  and  a  clear  portion 
siphoned  off.  Filtration  must  be  avoided,  since  contact  with 
filter  paper  almost  invariably  increases  the  acidity  of 
the  water. 


RELATIVE  ACIDITY  AND  ALKALINITY 


309 


Procedure  in  the  Case  of  a  Colourless  Water. — Measure 
25  c.c.  of  the  water  and  of  the  neutral  standard  into  two 
porcelain  basins  of  about  120  c.c.  capacity,  add  I  c.c.  of 
azolitmin  solution  to  each,  and  compare  the  tints. 

(a)  If  the  water  is  decidedly  acid,  measure  25  c.c.  of  the 
acid  phosphate  into  another  similar  basin,  add  the  indicator, 
and  run  in  the  alkaline  phosphate  from  a  burette  until  the 
colour  of  the  mixture  matches  that  of  the  water  sample 
with  indicator.  The  volume  of  the  water  sample  should  be 
kept  roughly  equal  to  the  volume  of  the  mixed  phosphates 
by  adding  more  of  the  sample  at  intervals.  Then  perform 
a  second  experiment  in  the  same  manner,  commencing, 
however,  with  a  volume  of  the  water  sample  such  that, 
when  a  good  match  is  obtained,  the  bulk  of  the  liquids  in 
the  two  basins  is  practically  the  same.  In  this  way,  the 
concentration  of  the  indicator  is  kept  the  same,  and  the  exact 
matching  of  the  tints  is  easier.  The  comparison  should  be 
made  in  good  diffused  daylight.  The  acidity  of  the  water, 
according  to  the  volume  of  alkaline  phosphate  required,  is 

given  in  Table  I. 

TABLE  I. 

For  decidedly  acid  waters. 
Volume  of  KH2PO4  used  =  25  c.c. 


Acidity. 

Volume  of  Na2HPO4 
required. 

Acidity. 

Volume  of  Na2HPO4 
required. 

300 

4-5 

8-07 

100 

0-28 

4 

9-15 

50 

0-63 

3-5 

10-5 

20 

1.68 

3 

12.3 

15 

2-23 

2'5 

15-2 

10 

3-57 

2 

19-2 

•      9 

3-97 

i-8 

21.4 

8 

4-52 

1-6 

24.2 

7 

5-18 

1.4 

27.7 

6 

6-05 

1-2 

32-7 

5 

7-26 

10* 

39-4 

Neutral. 


(ft)  If  the  water  is  alkaline  or  nearly  neutral,  measure 
25  c.c.  of  the  alkaline  phosphate  into  a  basin,  add  the 
indicator,  and  run  in  the  acid  phosphate  until  a  good  match 
is  obtained.  The  alkalinity  or  acidity  of  the  sample,  accord- 


310 


WATER  ANALYSIS 


ing  to  the  volume  of  acid   phosphate  required,  is  given  in 
Table  1 1. 

TABLE  II. 

For  nearly  neutral  and  alkaline  waters. 
Volume  of  Na2HPO4  used  --_-  25  c.c. 


Alkalinity. 

Volume  of  KH2PO4 
required. 

Acidity. 

Alkalinity. 

Volume  of  KH2PO4 
required. 

20 

3 

4-52 

15 

0-24 

2-5 

5-56 

10 

0-75 

2 

7-22 

9 

0.91 

1-8 

8-15 

8 

i-ii 

1-6 

9-30 

7 

1-39 

1.4 

10-8 

6 

1-77 

I  '2 

12-9 

5 

2-35 

I-O* 

I'O 

15-8 

4-5 

2.72 

1-2 

... 

19-1 

4 

3-15 

1-4 

22-6 

3-5 

3-74 

1-6 

... 

25-8 

D 


*  Neutral. 

Procedure  in  the  Case  of  a  Coloured  Water. — In  this 
case,  it  is  necessary  to  compensate  for  the  colour  of  the 
water  by  matching  the  solutions  in 
50  c.c.  Nessler  tubes.  The  tubes  are 
arranged  as  shown  in  Fig.  75.  The 
light  which  illuminates  the  phosphate 
standard  in  D  is  made  to  pass  also 
through  the  tube  B  containing  the 
water  sample  (without  indicator), 
whilst  the  light  which  reaches  the 
eye  through  the  water  sample  (with 
indicator)  in  C  passes  also  through 
a  column  of  distilled  water  in  A. 

Make  a  preliminary  titration  by 
the  first  method.  Suppose,  for  ex- 
ample, that  the  water  is  acid  and  is 
found  to  match  (approximately)  a 
mixture  of  25  c.c.  of  the  acid  phos- 
phate with  15  c.c.  of  the  alkaline 
phosphate  (total  volume  of  the  mix- 
ture =40  c.c.).  Measure  40  c.c.  of  distilled  water  into  the 
tube  A,  an  equal  volume  of  the  water  sample  into  the  tubes 


B 


FIG.  75- 

A  contains  distilled  water. 
B  contains  the  water  sample. 
C  contains  the  water  sample 

and  indicator. 
D  contains     the     phosphate 

standard  and  indicator. 


ACIDITY— LEAD  311 

B  and  C,  and  25  c.c.  of  the  acid  phosphate  into  the  tube  D, 
Add  I  c.c.  of  azolitmin  to  C  and  D,  and  run  the  alkaline 
phosphate  into  D  (mixing  the  solutions  by  means  of  a  glass 
tube  on  which  a  bulb  of  appropriate  size  is  blown)  until  the 
tints  in  C  and  D  match  exactly.  The  tubes  A  and  C  are 
held  in  the  left  hand  and  B  and  D  in  the  right,  and  the  two 
pairs  of  tubes  are  looked  into  from  above. 

Notes. — Hard  water — if  the  hardness  is  due  mainly  to 
bicarbonate — is  alkaline,  and  soft  water  is  often  acid.  If  the 
acidity  of  a  soft  water  containing  bicarbonate  is  due  entirely 
to  carbonic  acid,  the  water  will  become  alkaline  on  passing  a 
current  of  air,  free  from  carbon  dioxide,  through  the  water. 
The  acidity  must  be  due  to  some  acid  other  than  carbonic 
acid  if  this  treatment  does  not  remove  it.  It  should  be 
noted,  however,  that  a  water  from  which  the  carbon  dioxide 
has  been  entirely  removed  is  no  longer  in  equilibrium  with 
normal  air,  and  reabsorption  of  carbon  dioxide  commences 
immediately  the  water  is  exposed  to  ordinary  air. 

Lead. 

From  a  hygienic  standpoint,  it  is  most  important  that  a 
water  supply  should  not  dissolve  more  than  a  mere  trace  of 
lead  from  the  service  pipes.  The  action  of  a  water  on  lead 
is  closely  connected  with  its  effective  acidity.  Hard  water,  if 
the  hardness  is  due  to  bicarbonate,  is  alkaline,  and,  as  a  rule, 
its  only  action  is  to  coat  the  lead  pipe  with  an  insoluble  film 
which  adheres  to  the  pipe,  and  practically  no  lead  is  found  in 
the  water.  Soft  water  and  moorland  peaty  water,  which  are 
often  acid,  not  only  tarnish  lead  but  usually  dissolve  it,  and 
the  contamination  may  be  such  that  the  water  is  directly 
injurious  to  health.  On  the  other  hand,  some  very  soft 
waters,  such  as  the  Glasgow  water  supply,  are  without 
appreciable  solvent  action  on  lead. 

If  a  water  supply  is  suspected  of  causing  lead  poisoning, 
samples  should  be  taken  after  the  water  has  been  in  contact 
with  the  service  pipes  for  various  lengths  of  time,  up  to 
twelve  hours. 

Detection  of  Lead. — To  100  c.c.  of  the  water  add  2  c.c. 
of  acetic  acid  and  2  c.c.  of  hydrogen  sulphide  solution.  A 
brown  coloration  is  obtained  if  the  water  contains  more  than 


312  WATER  ANALYSIS 

o-oi  part  of  lead  in  100,000.  Iron  does  not  interfere  with 
the  test,  but  copper  gives  a  similar  coloration.  Copper  is, 
however,  rarely  present  in  drinking  water. 

A  very  much  smaller  quantity  of  lead  can  be  detected  in 
the  following  way  : — 

Filter  about  a  litre  of  the  water  through  a  small  plug  of 
pure  cotton  wool  placed  in  a  funnel.  Practically  all  the  lead 
salt  is  retained  by  the  cotton  wool.  Redissolve  the  lead  by 
pouring  hot  dilute  acetic  acid  over  the  cotton  wool,  and  then 
wash  with  distilled  water.  In  this  way  the  concentration  of 
the  lead  may  be  increased  almost  a  hundredfold.  Test  the 
solution  with  hydrogen  sulphide. 

Quantitative  Determination. — If  the  hydrogen  sulphide 
test  shows  the  presence  of  lead,  determine  the  amount 
colorimetrically,  as  described  on  p.  161. 

Action  of  Water  on  Lead. 

The  action  of  a  water  on  lead  may  be  determined  as 
follows : — 

Procure  some  new  J-inch  lead  service  piping,  and  cut  it 
into  lengths  of  2\  feet.  Close  one  end  of  each  piece  in  a 
vice.  The  capacity  of  the  tubes  so  formed  is  about  100  c.c. 

Carefully  rinse  out  two  or  three  of  the  tubes  with  the 
water  under  examination,  and  then  fill  the  tubes  completely 
with  the  water.  Fill  other  two  or  three  tubes  with  tap  water 
which  is  known  to  be  satisfactory  as  regards  its  action  on 
lead.  Cork  the  tubes  so  that  no  air  is  enclosed,  and  set  them 
aside  for  twenty-four  hours.  Then  empty  the  contents  of 
the  tubes  into  100  c.c.  Nessler  tubes,  and  determine  the  lead 
colorimetrically.  Repeat  the  experiment  until  practically 
constant  results  are  obtained. 

When  tested  in  this  way,  most  waters  will  dissolve  at 
least  a  trace  of  lead.  The  actual  amount  of  lead  found  in 
the  water  taken  from  the  tubes  will  probably  be  much  more 
than  would  be  present  in  a  domestic  water  supply  under 
ordinary  circumstances,  but  with  a  really  satisfactory  water 
it  should  not  be  more  than  about  o-i  part  of  lead  in  100,000. 
A  comparison  with  the  amount  of  lead  dissolved  in  the  tubes 
containing  the  tap  water  is  a  useful  guide. 


SALINE  CONSTITUENTS  313 

Iron. 

Natural  waters  containing  more  than  about  0-02  part  of 
iron  per  100,000  usually  become  opalescent  or  turbid  on 
exposure  to  the  air,  owing  to  the  decomposition  and  oxida- 
tion of  the  ferrous  hydrogen  carbonate.  The  precipitation  of 
the  iron  as  ferric  hydroxide  does  not  occur  so  readily  in  the 
case  of  a  water  containing  peaty  organic  matter. 

Quantitative  Determination. — To  100  c.c.  of  the  water, 
add  5  c.c.  of  concentrated  hydrochloric  acid  and  a  few  crystals 
of  potassium  chlorate,  and  evaporate  to  dryness.  Dissolve  the 
residue  in  i  c.c.  of  concentrated  hydrochloric  acid,  dilute  to 
100  c.c.,  and  determine  the  iron  colorimetrically  by  means  of 
ammonium  thiocyanate  (p.  156). 

Zinc  and  Copper. 

Zinc. — Water  which  has  been  in  contact  with  galvanised 
iron  pipes  may  become  contaminated  with  zinc.  Zinc  may 
be  detected  by  adding  to  100  c.c.  of  the  water  a  few  drops 
of  dilute  sulphuric  acid  and  I  c.c.  of  potassium  ferrocyanide 
solution.  If  01  part  of  zinc  per  100,000  is  present,  a  turbidity 
is  perceptible  in  about  one  minute  and  reaches  a  maximum 
after  about  five  minutes.  The  actual  amount  can  be  deter- 
mined by  comparing  the  turbidity  with  that  produced  by  a 
known  amount  of  zinc. 

Copper. — Copper  is  seldom  found  in  drinking  water.  It 
may  be  detected  and  colorimetrically  determined  by  means 
of  potassium  ferrocyanide  (p.  158).  Iron  interferes  with  the 
determination,  but  lead  does  not. 

DETERMINATION  OP  THE  SALINE  CONSTITUENTS. 

An  accurate  analysis  of  the  salts  dissolved  in  a  natural 
water  is  sometimes  required,  either  for  scientific  or  technical 
purposes.  The  analysis  involves  the  determination  of  silica, 
iron,  aluminium,  manganese  (rarely),  calcium,  magnesium, 
sodium,  and  potassium;  and  carbonate,  sulphate,  chloride, 
and  nitrate. 


314  WATER  ANALYSIS 

Determination  of  Silica,  Iron,  Aluminium, 
Calcium,  and  Magnesium. 

Measure  from  I  to  2  litres  of  the  water  into  a  large  silica 
flask,  add  10  c.c.  of  concentrated  hydrochloric  acid,  and  boil 
down  to  about  100  c.c.  A  large  Bunsen  flame  or  a  ring 
burner  may  be  used  at  first,  and  the  evaporation  of  a  litre  of 
water  need  not  take  longer  than  an  hour  and  a  half.  Transfer 
the  residual  solution  and  the  rinsings  of  the  flask  to  a  porcelain 
basin,  and  evaporate  to  complete  dryness  on  the  steam-bath. 

Determination  of  Silica. — Add  2  c.c.  of  concentrated 
hydrochloric  acid  to  the  dry  residue  and,  after  a  few  minutes, 
dilute  with  10  c.c.  of  water.  Warm  the  covered  basin  on  the 
steam-bath  for  several  minutes,  then  filter  through  a  small 
paper,  and  wash  the  insoluble  residue  with  warm  water  con- 
taining a  little  hydrochloric  acid.  Incinerate  the  paper  and 
ignite  the  insoluble  residue  in  a  platinum  crucible,  and  weigh. 
The  "  silica  "  must  be  examined  for  impurities  as  described  on 
p.  208.  It  may  contain  calcium  sulphate. 

Determination  of  Iron,  Aluminium,  Calcium,  and 
Magnesium. — To  the  filtrate  from  the  silica,  contained  in  a 
porcelain  (not  platinum)  basin,  add  a  few  drops  of  con- 
centrated nitric  acid,  evaporate  to  dryness,  and  ignite  gently 
in  order  to  destroy  organic  matter.  Moisten  the  residue 
with  2  c.c.  of  concentrated  nitric  acid,  add  10  c.c.  of  hot 
water,  filter,  and  wash  with  warm  water  containing  a  little 
nitric  acid.  Determine  the  iron,  aluminium,  calcium,  and 
magnesium  as  described  under  "Analysis  of  Dolomite" 

(p.  228). 

Determination  of  Sodium  and  Potassium, 

Evaporate  a  measured  volume  of  the  water,  in  the  manner 
already  described,  to  about  100  c.c.  Transfer  to  a  platinum 
basin  and  concentrate  to  50  c.c.  To  the  hot  solution  add 
a  slight  excess  of  a  hot,  saturated  solution  of  barium 
hydroxide,  in  order  to  precipitate  iron,  magnesium,  sulphate, 
etc.  Evaporate  until  the  volume  of  the  liquid  is  about 
25  c.c.,  then  filter  the  precipitate  and  wash  it  with  hot  water. 
The  filtrate  contains  barium,  calcium,  sodium,  and  potassium. 


SIGNIFICANCE  OF  RESULTS  315 

Remove  the  calcium  and  barium,  and  determine  the  sodium 
and  potassium,  as  described  on  p.  235. 

Determination  of  Sulphate,  Carbonate,  Chloride,  and 
Nitrate. 

Sulphate. — Determine  whether  much  sulphate  or  only 
a  small  amount  is  present  by  adding  one  drop  of  dilute 
hydrochloric  acid  and  a  few  drops  of  barium  chloride  solu- 
tion to  10  c.c.  of  the  water;  10  parts,  or  more,  of  sulphate 
(SO4)  in  100,000  give  an  immediate  precipitate. 

Take  200  c.c.  of  the  water  (or  more  if  the  precipitate  with 
barium  chloride  is  small  and  forms  slowly),  add  about  2  c.c. 
of  dilute  hydrochloric  acid,  and  evaporate  to  about  50  C.C.1 
Determine  the  sulphate  as  barium  sulphate  (p.  131). 

Carbonate. — The  total  "alkalinity"  of  the  water,  as 
determined  by  titration  with  hydrochloric  acid  and  described 
under  "  Hardness,"  gives  the  amount  of  carbonate  present. 

One  c.c.  of  0-02  N  acid  corresponds  to  0-6  mgrm.  CO3, 
and  if  100  c.c.  of  the  water  is  titrated,  each  cubic  centimetre 
of  the  acid  required  corresponds  to  0-6  part  of  carbonate  in 
100,000. 

Chloride  and  Nitrate. — The  determination  of  chloride  and 
of  nitrate  has  been  already  described. 

SIGNIFICANCE   OP   THE  RESULTS    OP    ANALYSIS   IN 
THE   CASE  OP   A   POTABLE   WATER. 

The  purity  of  a  drinking  water  is  judged  mainly  from  the 
amounts  of  nitrogenous  substances  (free  and  albumenoid 
ammonia,  nitrite,  and  nitrate),  organic  matter  (absorption  of 
oxygen),  and  chloride  which  it  contains.  The  question 
usually  resolves  itself  into  ascertaining  from  the  results  of 
analysis  —  considered,  not  individually  but  collectively  — 
whether  the  water  has  been  polluted,  and  whether  the  water, 
in  its  present  condition,  is  fit  to  drink. 

It   is   practically  impossible  to  fix  definite   standards  of 

purity  based  on  the  amounts  of  any  of  the  above  substances. 

A  water  must  be  judged  on  its  own  merits,  and  with  full 

knowledge   of  the   source   from   which   it   is   derived.      No 

1  See  footnote  2  on  p.  304. 


316  WATER  ANALYSIS 

attempt  has  been  made  to  discuss  the  full  significance  of  the 
results  of  analysis.  A  few  notes  are  given  below,  and  further 
information  will  be  found  in  Thresh's  Examination  of  Water 
and  Water  Supplies  (J.  and  A.  Churchill). 

Residue  on  Evaporation. — It  is  desirable  that  this  should 
not  exceed  50  parts  per  100,000. 

Hardness. — If  the  total  hardness  is  under  5°,  the  water 
may  be  considered  "soft";  from  5°  to  10°,  "fairly  soft"; 
from  10°  to  20°,  "hard  ";  and  over  20°,  "very  hard."  If  the 
hardness  exceeds  30°,  the  water  is  very  unsuitable  for 
general  purposes. 

Chloride. — Sewage  contains  chloride,  and  the  presence  of 
much  chloride  in  a  water  (especially  if  accompanied  by  an 
excessive  amount  of  nitrate)  may  be  an  index  of  pollution — 
provided  the  chloride  is  not  derived  from  sea-water  or  other 
natural  source.  As  a  general  rule,  a  surface  water  contains 
less  than  2  parts  of  chloride  (Cl)  in  100,000.  Well  water 
may  contain  much  more. 

Free  and  Albumenoid  Ammonia.  —  Decaying  organic 
matter  and  sewage  contain  ammonia,  and  the  presence  of 
more  than  about  0-005  part  of  ammonia  in  100,000  is  signifi- 
cant of  possible  pollution.  If  a  water  yields  more 
free  ammonia  than  albumenoid  ammonia,  the  nitrogenous 
impurity  is  probably  derived  from  sewage.  On  the  other 
hand,  slight  sewage  pollution — sufficient  to  render  a  water 
unsafe — cannot  always  be  detected  by  chemical  methods. 
Peaty  water  yields  the  albumenoid  ammonia  slowly,  and 
usually  in  larger  amount  than  the  free  ammonia. 

A  satisfactory  water  seldom  yields  as  much  as  o-oi  part  of 
albumenoid  ammonia  per  100,000. 

Reducing  Power. — In  the  case  of  an  upland  surface  water, 
the  absorption  of  less  than  o-i  part  of  oxygen  per  100,000 
and  of  less  than  0-05  in  the  case  of  other  waters,  is  usually 
regarded  as  indicating  waters  of  great  organic  purity.  The 
result  of  any  one  determination  is,  however,  rarely  sufficient 
to  condemn  a  water  or  to  justify  its  use,  and  the  amount 
of  oxygen  absorbed  should  be  considered  together  with  the 
yield  of  albumenoid  ammonia. 

Nitrite. — Nitrite  may  arise  from  the  reduction  of  nitrate, 


SIGNIFICANCE  OF  RESULTS  317 

but  it  occurs  in  sewage  and  manure,  and  it  should  be  entirely 
absent  from  a  drinking  water. 

Nitrate. — Nitrate,  like  chloride,  is  innocuous  ;  but,  as  it 
is  derived  from  nitrogenous  organic  matter  of  animal  origin 
and  is  the  final  product  of  its  decomposition,  its  presence  in 
a  water  is  an  index  of  past  pollution.  It  is  not  possible  to 
fix  a  standard  of  purity  in  respect  of  nitrate,  but  a  satis- 
factory surface  water,  even  from  cultivated  land,  seldom 
contains  more  than  i  part  of  nitrate  (NO3)  in  100,000.  Well 
water  often  contains  a  larger  amount. 

Acidity  or  Alkalinity. — If  the  relative  acidity  of  a  water 
is  three  or  four  times  that  of  ideally  pure  water,  the  solvent 
action  of  the  water  on  lead  requires  careful  investigation. 

Lead. — Drinking  water  should  be  entirely  free  from  lead. 
On  account  of  the  cumulative  nature  of  the  poison,  it  is 
difficult  to  fix  a  limit  to  the  amount  of  lead  that  is  permissible 
in  a  water  supply.  If  the  amount  never  exceeds  0-03  part 
per  100,000,  the  water  may  probably  be  regarded  as  safe. 
A  water  supply  which,  under  normal  circumstances,  is  liable 
to  contain  up  to  o-i  part  of  lead  per  100,000  must  be 
described  as  dangerous. 


PART    IX 

QUANTITATIVE    ANALYSIS   OF 
ORGANIC   SUBSTANCES 

IN  the  analysis  of  organic  compounds,  the  chief  elements 
of  importance  which  have  to  be  considered  are  carbon, 
hydrogen,  oxygen,  nitrogen,  the  halogens,  and  sulphur. 

Carbon  and  hydrogen  are  determined  simultaneously 
after  oxidation  to  carbon  dioxide  and  water ;  nitrogen  is 
obtained  and  measured  in  the  free  state,  or  is  converted 
into  ammonia;  the  halogens  are  converted  into  the  corre- 
sponding silver  or  sodium  salts ;  sulphur  is  determined, 
after  oxidation,  as  barium  sulphate.  There  is  no  direct 
method  for  determining  oxygen. 

If  metallic  radicals  are  present,  they  are  determined  after 
oxidising  the  organic  matter.  The  following  method  of 
oxidation  is  generally  applicable.  To  a  weighed  quantity 
(about  2  grams)  of  the  substance  in  a  porcelain  basin,  10 
c.c.  of  concentrated  sulphuric  acid  are  added.  The  mixture 
is  well  stirred,  and  is  heated  on  the  steam-bath  for  ten 
minutes.  A  few  drops  of  concentrated  nitric  acid  are  then 
added,  the  mixture  is  heated  on  a  sand-bath  until  it  begins 
to  fume,  and,  at  intervals  of  a  few  minutes,  two  or  three 
drops  of  nitric  acid  are  added.  When  the  solution  has 
become  colourless,  it  is  heated  until  it  fumes  strongly;  it 
is  then  cooled,  and  the  metallic  radicals  are  determined  in 
the  usual  manner. 

CARBON   AND   HYDROGEN. 

The  determination  of  carbon  and  hydrogen  is  accomplished 
by  a  process  of  oxidation  in  which  a  known  weight  of  the 

318 


COMBUSTION  APPARATUS 


319 


substance  is  burned  in  a  current  of  air  or  oxygen  and  in 
presence  of  copper  oxide.  The  carbon  and  hydrogen  are 
quantitatively  oxidised  to  carbon  dioxide  and  water,  which 
are  separately  collected  and  weighed.  The  whole  process  is 
termed  a  "combustion." 

The  following  apparatus  is  required  : — 

(1)  A  Combustion  Tube  of  Jena  glass,  70  cm.  long  and 
8  mm.  internal  diameter.     The  sharp  internal  and  external 
edges  at   each   end   of  the   tube   are   rounded   by  heating 
carefully   in   a   blowpipe   flame.      The   tube   is   cleaned   by 
drawing  through  it  a  plug  of  moist   cotton  wool  attached 
to  a  piece  of  string,  and  then  rinsing  with  water.     It  is  dried 
by  warming  and  blowing  a  current  of  air  through  it. 

(2)  A  Heating  Furnace  of  the  Dennstedt  type,  shown  in 
Fig.  83.     It  consists  essentially  of  an  iron  trough  or  gutter, 
60  cm.  long,  supported  on  two  uprights.     The  gutter  is  lined 
with  asbestos  cloth  or  fibre,  and  the  combustion  tube,  which 
is  laid  in  the   gutter,  is   heated   by   three   Bunsen   burners 
provided   with   flame   spreaders.      Iron    covers,   lined    with 
asbestos,  and  supported  on  two  angle  irons,  are  placed  over 
the  tube  during  the  combustion  process. 


Glass  Woolp 

Calcium 
Chloride 


Sulphuric 
Acid 


FIG.  76. 


(3)  A   Purifying    Tower    for    removing    moisture    and 
carbon  dioxide  from  the  oxygen.     This  is  shown  in  Fig.  76, 


320 


ANALYSIS  OF  ORGANIC  SUBSTANCES 


and  should  be  about  30  cm.  high  and  10  cm.  in  diameter. 
The  oxygen  enters  at.  F  and  passes,  first  through  sulphuric 
acid,  next  over  granular  soda-lime  in  order  to  remove 
carbon  dioxide,  and  then  over  granular  calcium  chloride. 

(4)  A  Gas-holder  of  about  5   litres   capacity  containing 
oxygen.     The  gas-holder  (Fig.  77)  is  most  conveniently  filled 
from    a    cylinder  of  compressed   oxygen. 
The    gas    must    be    free    from   traces    of 
hydrogen. 

(5)  Apparatus  for  the  Absorption  of 
Water  and  Carbon  Dioxide. 

(a)  The  water  formed  in  the  com- 
bustion is  absorbed  in  either  calcium 
chloride  or  concentrated  sulphuric  acid. 
The  most  convenient  form  of  calcium 
chloride  tube  is  shown  in  Fig.  49,  on 
p.  177.  The  tube  is  filled  and  any  free 
lime  or  basic  chloride  in  the  calcium 
chloride  removed  by  treatment  with  car- 
bon dioxide,  as  described  on  p.  176.  The 
carbon  dioxide  in  the  tube  is  displaced 
by  passing  dry  oxygen  through  it  for  ten 
minutes,  and  the  taps  are  then  closed. 

A  rubber  cap  (or  short  piece  of  rubber  tubing  fitted  with 
a  plug  of  glass  rod)  should  be  provided  for  the  purpose  of 
closing  the  bulbed  side-tube. 

If  sulphuric  acid  is  to  be  used 
for  the  absorption  of  water,  an 
unstoppered  U-tube  is  required 
(Fig.  78).  A  quantity  of  granular 
pumice  sufficient  to  fill  the  tube 
is  soaked  in  concentrated  sul- 
phuric acid  for  a  short  time,  the 
excess  of  acid  is  drained  off,  and 
the  pumice,  contained  in  a  por- 
celain basin,  is  heated  in  a  good 
draught  until  fuming  nearly  ceases. 
The  U-tube  is  then  filled  with 
the  pumice  to  within  I  cm.  of  the  side-tubes,  and  the  tube 
is  sealed  in  the  blowpipe  flame  at  a  and  b.  Concentrated 


FIG.  77. 


FIG.  78. 


COMBUSTION  APPARATUS  321 

sulphuric  acid  is  then  drawn  into  it  through  the  shorter  side- 
tube  and,  after  the  acid  has  been  in  contact  with  the  pumice 
for  about  ten  minutes,  the  excess  of  acid  is  drained  off  until 
the  quantity  that  remains  is  no  more  than  sufficient  to  fill  the 
bend  of  the  U-tube.  The  side-tube  that  has  been  in  contact 
with  the  acid  is  then  gently  heated  with  a  small  flame  until 
the  acid  that  wets  it  is  volatilised.  The  air  in  the  tube  is 
displaced  by  passing  a  current  of  dry  oxygen  for  five 
minutes,  and  the  tube  is  then  closed  with  caps,  fitted  over 
the  side-tubes  and  made  from  short  pieces  of  rubber  tubing 
closed  with  plugs  of  glass  rod. 

(U)  For  the  absorption  of  the  carbon  dioxide  formed  in 
the  combustion,  two  U-tubes  (see  Fig.  50,  on  p.  177)  are 
required.  A  small  wad  of  cotton  wool  is  placed  near  the 
middle  of  one  limb,  and  fine  granular  soda-lime  is  introduced 
so  as  to  fill  about  three-fourths  of  the  tube.  The  remaining 
fourth  is  filled  with  granular  calcium  chloride,  and  small 
wads  of  glass  wool  are  placed  in  each  limb.  The  taps  are 
made  gas-tight  with  the  minimum  quantity  of  grease,  and 
the  air  in  the  tubes  is  displaced  by  passing  a  current  of  dry 
oxygen  for  five  minutes. 

After  the  absorption  tubes  are  filled,  they  are  wiped  with 
a  dry  cloth,  care  being  taken  to  remove 
any  grease  exposed  at  the  taps,  and  are 
then  placed  in  a  cardboard  box  (Fig.  79) 
and   taken   to    the   balance  -  room,   where 
they  should  remain  for   at  least   half  an 
hour  before  weighing.     Equilibrium  with 
the  moisture  in  the   surrounding  air  will          "  FlG 
usually  be  more  quickly  established  if  the 
tubes,  after  wiping,  are  lightly  breathed  upon. 

If  the  balance  pan  is  large  enough,  the  tube  may  be 
laid  upon  it  while  the  weighing  is  in  progress,  or  it 
may  be  suspended  from  the  hook  of  the  balance  by 
means  of  a  stirrup  made  of  aluminium  wire,  the  com- 
bined weight  of  the  tube  and  stirrup  being,  of  course, 
recorded.  If  the  pumice  and  sulphuric  acid  U-tube  is 
used  for  the  absorption  of  water,  it  must  be  suspended  in 
this  way.  Before  weighing,  the  rubber  caps  are  removed 
from  the  side-tubes. 

x 


322  ANALYSIS  OF  ORGANIC  SUBSTANCES 

(6)  A  Pulsimeter,  or  indicator  (Fig.  80),  containing  a 
few  drops  of  concentrated  sulphuric  acid,  is  used  to  show 
the  rate  at  which  unabsorbed  gas  leaves 
the  apparatus  and,  at  the  same  time, 
to  protect  the  calcium  chloride  in  the 
last  absorption  tube  from  atmospheric 
moisture. 

(7)  A  Tube  (Fig.  81,   A),  about  20 
cm.  long  and  2  cm.  diameter,  provided 
with  a  cork  and  calcium  chloride  tube. 
FIG.  80.  A  constriction,  into  which  the  neck  of 

the  weighing-tube  B  exactly  fits,  is  made  near  the  open  end 
of  the  tube  A. 

A 


B 

FIG.  8r. 

(8)  A  Stoppered  Weighing-tube  (Fig.  81,  B),  about  5  cm. 
long,  the  neck  of  which  is  of  such  a  diameter  as  to  fit  into 
the  end  of  the  combustion  tube.  This  tube  may  be  made 
from  a  piece  of  glass  tubing,  and  the  stopper  from  a  piece 
of  glass  rod  ground  into  the  neck  by  means  of  emery  or 
carborundum  powder. 

Preparation  of  the  Combustion  Tube. 

A  quantity  of  wire-form  copper  oxide,  sufficient  to  fill 
the  combustion  tube  (30  to  35  grams),  is  carefully,  broken 
(not  ground)  in  a  mortar  into  somewhat  smaller  pieces,  and 
is  then  sifted  from  dust  through  a  3o-mesh  sieve. 

Another  portion  (about  15  grams)  of  the  oxide  is  crushed 
until  it  is  fine  enough  to  pass  through  a  3O-mesh  sieve.  This 
powder  is  then  sifted,  by  means  of  a  6o-mesh  sieve,  from  fine 
dust,  and  the  latter  is  rejected. 

A  small  plug  of  asbestos  is  placed  in  the  combustion  tube 


PREPARATION  OF  COMBUSTION  TUBE  323 

about  6  cm.  from  one  end.  The  tube  is  then  charged  with 
the  coarse  copper  oxide  until  it  is  about  two-thirds  full 
(44  cm.),  and  another  asbestos  plug  is  used  to  keep  the 
oxide  in  place.  Fine  copper  oxide  is  then  introduced  until 
the  tube  is  filled  to  within  5  cm.  of  the  end.  The  ends  of 
the  tube  are  fitted  with  rubber  stoppers  through  which  pass, 
at  the  end  E,  a  short  length  of  capillary  (i  mm.  bore)  glass 
tubing,  and  at  the  end  D,  a  small  straight  calcium  chloride 
tube.  The  combustion  tube  is  then  laid  in  the  furnace,  and 
the  end  E  is  connected  with  the  drying  tower  and  oxygen 
supply  by  means  of  rubber  tubing. 

c  8 

Fine   Copper     £ 

Oxide    and      4  Coarse  "8 

Substance       <  Copper  Oxide  < 

1       i                                ,  '        ' 

5cm.   < — 14 cm. — >     '         < 44cm-~         — *          5cm. 

EC  D 

FIG.  82. 

The  gas  burners  are  lighted,  small  flames  being  used  at 
first,  and,  the  covers  having  been  laid  in  position,  the  tube  is 
gradually  heated  to  low  redness,  and  a  slow  current  of  oxygen 
is  passed  through  it.  Care  must  be  taken  not  to  char  the 
rubber  stoppers,  and,  in  order  to  protect  them  from  the 
heat,  two  pieces  of  asbestos  board  provided  with  a  hole  or 
slit  may  be  slipped  over  the  ends  of  the  tube.  If  any 
moisture  condenses  near  the  end  D,  the  asbestos  screen  is 
removed  for  a  time,  or  the  end  of  the  tube  is  gently  warmed 
with  a  small  flame  until  the  moisture  disappears.  After  the 
tube  has  been  heated  for  half  an  hour,  the  oxygen  current  is 
stopped  and  the  tube  allowed  to  cool.  The  fine  oxide  in  the 
rear  of  the  tube  is  then  transferred  to  the  tube  A,  and  the 
latter  is  at  once  closed  with  the  calcium  chloride  tube.  The 
combustion  tube,  which  is  now  ready  for  use,  is  at  once 
attached  to  the  drying  tower,  or  a  calcium  chloride  tube  is 
fitted  into  the  end  E. 


324  ANALYSIS  OF  ORGANIC  SUBSTANCES 

Combustion  of  a  Solid  Substance  containing  Carbon  and 
Hydrogen  only,  or  Carbon,  Hydrogen,  and  Oxygen. 

The  substance  to  be  analysed  must  be  perfectly  dry.  A 
small  quantity,  from  0-15  to  0-2  gram,  is  accurately  weighed 
in  the  small  stoppered  tube  B  (Fig.  81).  The  substance  is 
mixed  in  the  tube  B  with  a  small  quantity  of  copper  oxide 
from  A,  and  the  mixture  is  transferred  to  the  combustion 
tube.  The  tube  is  then  "washed  out"  several  times  with 
copper  oxide,  received  as  before  from  A,  and  emptied  into 
the  combustion  tube,  care  being  taken  that  nothing  is  lost  in 
the  process,  and  that  the  copper  oxide  is  exposed  to  the 
atmosphere  as  little  as  possible.  The  mixture  of  copper 
oxide  and  substance  should  fill  about  14  cm.  of  the  tube, 
and  should  be  kept  in  place  by  means  of  a  short  roll  of 
copper  gauze.  About  5  cm.  of  the  tube  remain  unoccupied. 

The  following  method  of  mixing  the  substance  with  fine 
copper  oxide  in  the  combustion  tube  is  also  convenient : — 

A  small  quantity  of  copper  oxide  is  first  shaken  into  the 
combustion  tube  from  the  tube  A,  and  the  substance  is 
introduced  from  the  tube  B  (which  is  afterwards  weighed 
again).  More  copper  oxide  is  then  added,  in  small  portions 
at  a  time,  and,  after  each  addition,  the  substance  and  copper 
oxide  are  mixed  by  rotating  the  combustion  tube. 

The  combustion  tube  is  now  laid  in  the  furnace,  and  the 
end  E  is  connected  as  before  with  the  oxygen  supply.  The 
calcium  chloride  tube  is  removed  from  the  end  D,  and  the 
weighed  absorption  tubes  are  attached.  The  tubes  are 
suspended,  by  means  of  hooks  made  of  stout  wire,  from  a 
rod  fixed  horizontally  in  a  clamp.  A  rubber  stopper,  which 
must  fit  the  combustion  tube  accurately ',  is  pushed  on  to  the 
bulbed  side-tube  of  the  calcium  chloride  u-tube  (or  the  con- 
centrated sulphuric  acid  U-tube),  until  the  end  of  the  side- 
tube  is  flush  with  the  end  of  the  stopper.  The  stopper  is 
then  tightly  fixed  into  the  combustion  tube.  (It  is  a  good 
plan  to  lubricate  the  hole  in  the  stopper  by  rubbing  powdered 
graphite  into  it  with  a  thin  glass  rod,  the  loose  graphite 
being  carefully  removed  ;  the  stopper  will  then  slip  on  to  the 
U-tube  easily,  and  is  easily  removed  again  when  the  com- 
bustion is  over.  A  trace  of  graphite  rubbed  on  the  surface 
of  the  stopper  prevents  it  sticking  to  the  combustion  tube.) 


COMBUSTION  OF  A  SOLID 


325 


The  two  soda-lime  tubes  are  next  attached,  the  limbs 
containing  calcium  chloride  being  in  each  case  turned 
away  from  the  combustion  tube.  The  connections  are 
made  with  short  pieces  of  thick  -  walled  rubber  tubing 
(pressure  tubing)  lubricated  with  graphite,  and  the  ends  of 
the  glass  tubes  should  be  brought  close  together  inside  the 
rubber  junction.  Wiring  is  unnecessary  and  should  not  be 
resorted  to  as  a  means  of  making  the  joints  gas-tight.  The 
pulsimeter  is  finally  attached  in  the  same  way,  and  the 
combustion  proper  commenced. 

The  taps  of  the  (j-tubes  are  opened  and  a  current  of 
oxygen  is  started  at  the  rate  of  about  one  bubble  per  second, 


FlG.  83. — General  Arrangement  of  Combustion  Apparatus. 

as  seen  in  the  pulsimeter.  The  burners  under  the  front  portion 
(Fig.  82,  CD)  of  the  tube  are  lighted,  covers  are  placed  over 
this  part  of  the  tube,  and  the  temperature  is  gradually  raised. 
The  back  portion  of  the  tube  is  meanwhile  protected  from 
the  heat  as  far  as  possible  by  means  of  screens  of  asbestos 
board  or  paper.  When  the  copper  oxide  has  attained  dull 
redness,  the  small  burner  at  the  other  extremity  of  the  tube 
is  lighted,  and  without  using  a  cover  at  this  stage.  The 
flame,  which  is  kept  small  at  first,  is  gradually  increased,  and 
the  burner  is  slowly  moved  forward,  covers  being  placed 
over  the  tube  behind  the  burner  in  order  to  keep  the  back 
portion  of  the  tube  hot.  The  substance  burns  for  the  most 
part  in  the  moderately  rapid  current  of  oxygen,  and,  pro- 
vided sufficient  oxygen  is  supplied,  there  is  often  little  or  no 
visible  reduction  of  copper  oxide  to  metallic  copper.  The 
first  indication  that  combustion  is  taking  place  is  the  appear- 
ance of  moisture  at  the  front  of  the  combustion  tube,  and, 
somewhat  later,  the  heat  that  develops  in  the  first  soda-lime 
tube.  The  more  volatile  the  substance  subjected  to  com- 


326  ANALYSIS  OF  ORGANIC  SUBSTANCES 

bustion,  the  greater  the  care  necessary  in  heating  the  back 
portion  of  the  tube.  If  the  heating  is  too  rapid,  incomplete 
combustion  may  result.  A  rush  of  gas  through  the  pulsi- 
meter  must  be  at  once  checked  by  removing  the  flame  for  a 
time. 

When  the  whole  of  the  back  portion  of  the  tube  has  been 
carefully  heated  in  this  way,  all  the  covers  are  laid  in  position, 
a  flame  spreader  is  placed  on  the  back  burner,  and  the 
whole  tube  is  heated  as  uniformly  as  possible  to  dull  redness. 
Any  carbon  that  may  have  been  formed  by  the  decomposition 
of  the  substance  is  thus  burned  away.  The  temperature  of 
the  tube  near  the  front  cork  must  be  carefully  regulated  with 
the  help  of  the  asbestos  screen  ;  if  moisture  tends  to  collect 
there,  the  screen  should  be  removed  for  a  time,  or  the  tube 
warmed  with  a  small  flame  until  the  moisture  disappears. 

Finally,  when  oxygen  passes  freely  through  the  pulsi- 
meter  and  when  the  first  soda-lime  tube  is  practically  cold, 
the  combustion  is  finished.  The  burners  are  then  ex- 
tinguished and  the  absorption  tubes  detached.  The  taps 
are  closed,  and  the  rubber  cap  is  replaced  on  the  calcium 
chloride  U-tube.  The  tubes  are  carefully  wiped  with  a  dry 
cloth,  and,  after  an  interval  of  not  less  than  half  an  hour, 
are  weighed.  The  fine  copper  oxide  in  the  back  portion  of 
the  tube  is  transferred  to  the  tube  A,  and  the  combustion 
tube  is  at  once  closed  at  each  end  with  a  calcium  chloride 
tube  (the  rear  end  may  be  left  attached  to  the  drying  tower). 
The  tube  is  then  ready  for  the  next  combustion. 

Example — 

Weight  of  substance  taken  =0-1521  gram 

Increase  in  weight  of  calcium  chloride  tube       =  0-0684      >» 
Increase  in  weight  of  1st  soda-lime  tube  =  0-3828      „ 

Increase  in  weight  of  2nd  soda-lime  tube  =  0-0004      „ 

Percentage  of  carbon  =  0-3832  x  -5-  x     IQO      =    68-70 

C7H6O2  (benzoic  acid)  requires  C    =    68-82 

Difference          =  —0-12 

Percentage  of  hydrogen  =  0-0684  x  -    -  X  — =  5-04 

C7H6O2  requires         H  =      4-96 
Difference        =  +0-08 


COMBUSTION  OF  A  LIQUID  327 

As  a  rule  the  percentage  of  carbon  found  in  a  pure  substance 
is  a  little  below  and  that  of  hydrogen  a  little  above  the 
calculated  values.  The  difference  in  each  case  should  not 
exceed  about  o-i  per  cent. 

The  calcium  chloride  U-tube  may  be  used  for  a  large 
number  of  combustions  without  renewing  the  contents.  The 
water  which  collects  in  the  bulb  should  be  drained  off  from 
time  to  time.  The  weight  of  the  second  soda-lime  tube 
should  remain  practically  constant.  If  a  decided  increase 
in  the  weight  of  the  second  soda-lime  tube  is  observed,  the 
soda-lime  (but  not  the  calcium  chloride)  in  the  first  of  these 
tubes  should  be  renewed. 

Combustion  of  a  Liquid. 

The  liquid  is  weighed  in  a  small  bulb  (Fig.  84)  about 
2  cm.  long,  provided  with  a  fairly  wide  capillary  8  cm.  long. 
The  liquid  is  introduced  by  warming  the  weighed  bulb,  and 


-3cm, 


FIG.  84. 

then  allowing  it  to  cool  with  the  open  capillary  immersed  in 
the  liquid.  Another  convenient  method  is  to  place  the  bulb- 
tube,  with  the  capillary  immersed  in  the  liquid,  in  a  desic- 
cator, which  is  then  evacuated.  On  re-admitting  air  to  the 
desiccator,  the  liquid  is  forced  into  the  bulb.  The  capillary  is 
gently  warmed  in  order  to  drive  out  the  liquid  which  it 
contains,  and  is  then  sealed.  The  tube  and  contents  are  then 
weighed.  In  the  final  weighing  and  subsequent  handling  of 
the  bulb,  care  should  be  taken  to  prevent  the  liquid  re- 
entering  the  capillary. 

A  file  scratch  is  made  near  the  tip  of  the  capillary,  the 
tip  is  broken  off,  and  the  bulb-tube,  surrounded  by  copper 
oxide  (received  from  the  tube  A,  Fig.  Si,  on  p.  322),  is  placed 
in  the  back  part  of  the  combustion  tube  with  the  open 
end  of  the  capillary  facing  the  current  of  oxygen.  The 


328  ANALYSIS  OF  ORGANIC  SUBSTANCES 

combustion  is  started  in  the  usual  way  and,  when  the 
copper  oxide  in  the  front  of  the  tube  is  hot,  the  liquid  is 
slowly  distilled  out  of  the  bulb  into  the  copper  oxide  at  the 
cool  end  of  the  tube  by  means  of  a  small  flame  applied 
directly  to  the  upper  side  of  the  combustion  tube  and  over 
the  bulb.  The  combustion  is  then  continued  as  for  a  solid. 

In  the  case  of  a  decidedly  volatile  liquid,  such  as  benzene, 
a  bulb  with  a  capillary  at  each  end  is  used.  The  longer 
capillary  (8  cm.)  is  plugged  with  fusible  metal  and  the 
shorter  (3  cm.)  is  sealed  off  in  the  usual  way  after  the 
liquid  has  been  introduced.  Before  the  bulb  is  put  into  the 
combustion  tube,  the  absorption  tubes  are  attached  and  the 
front  of  the  tube  is  heated  to  dull  redness.  The  bulb  is 
then  pushed  into  the  cool  part  of  the  tube  with  the  longer 
capillary  facing  the  current  of  oxygen.  The  plug  of  fusible 
metal  is  then  melted  by  the  application  of  a  small  flame  above 
the  combustion  tube,  and  the  liquid  is  gradually  vaporised 
by  very  gentle  heating. 

Modification  of  the  Process  if  Nitrogen  is  Present. 

If  the  substance  contains  nitrogen,  the  greater  part  of 
that  element  is  liberated  in  the  free  state  and  escapes 
through  the  absorption  tubes.  Traces  of  nitrogen  oxides, 
however,  may  be  produced,  and,  in  order  to  decompose  these 
oxides  and  thus  prevent  them  reaching  the  absorption  tubes, 
a  closely  wound  roll  of  copper  gauze,  7  cm.  long,  is  placed 
in  front  of  the  copper  oxide  in  the  combustion  tube ;  some  of 
the  copper  oxide  is  removed  to  make  room  for  the  roll, 
which  must  lie  well  within  the  furnace.  A  clean,  metallic 
surface  is  obtained  by  heating  the  roll  to  redness  in  a 
large  flame,  and  dropping  it  quickly  into  a  test-tube  contain- 
ing about  0-5  c.c.  of  methyl  alcohol.  The  roll  is  then  dried 
for  not  more  than  ten  minutes  in  the  steam-oven,  and  kept 
in  a  desiccator  until  placed  in  the  combustion  tube.  The 
roll  must  be  heated  to  low  redness  before  the  substance 
begins  to  burn.  In  the  early  stages  of  the  combustion,  the 
current  of  oxygen  in  this  case  should  be  passed  at  a  somewhat 
slower  rate  than  usual. 


NITROGEN  BY  DUMAS'  METHOD  329 

Modification  if  Sulphur  or  a  Halogen  is  Present. 

If  the  substance  contains  a  halogen  or  sulphur,  combustion 
with  copper  oxide  will  yield  volatile  or  unstable  compounds 
of  copper  (copper  halides  and  copper  sulphate),  and  free 
halogen  or  sulphur  dioxide  will  reach  the  absorption  tubes 
and  spoil  the  analysis.  In  both  cases  the  best  method  is 
to  use  fused,  granulated,  lead  chromate  instead  of  copper 
oxide  in  the  combustion  tube.  The  lead  halides  and  lead 
sulphate  are  more  stable  and  less  volatile  than  the  corre- 
sponding copper  salts ;  but  the  temperature,  especially  in 
the  extreme  front  portion  of  the  tube,  must  not  be  too  high. 
In  the  case  of  a  halogen  compound,  a  roll  of  silver  gauze, 
placed  in  front  of  the  copper  oxide  and  kept  at  a  moderate 
temperature,  will  usually  retain  any  halogen. 

NITROGEN. 

Two  methods  are  in  common  use  for  the  determination 
of  nitrogen  in  organic  substances:  (i)  Dumas'  method  of 
combustion  with  copper  oxide,  in  which  the  nitrogen,  evolved 
as  gas,  is  collected  and  measured  ;  (2)  Kjeldahl's  method,  in 
which  the  substance  is  decomposed  by  heating  with  con- 
centrated sulphuric  acid,  whereby  the  nitrogen  is  converted 
into  ammonium  sulphate  and  is  then  determined  as  ammonia 
by  the  usual  volumetric  method.  The  first  method  is  applic- 
able to  practically  all  types  of  organic  compounds,  and 
is  generally  preferred  for  scientific  purposes.  Kjeldahl's 
method  is  suitable  only  for  compounds  in  which  the  nitro- 
gen is  directly  linked  with  carbon  and  hydrogen,  and  cannot 
be  used  for  nitro-,  nitroso-,  or  azo-compounds  unless  these 
receive  adequate  preliminary  treatment.  The  method  is 
widely  employed  in  commercial  analysis. 

Nitrogen  by  Dumas'  Method. 

The  following  apparatus  is  required  : — 

(i)  A  Combustion  Furnace  and  Combustion  Tube  similar 
to  those  used  for  the  determination  of  carbon  and  hydrogen. 
The  combustion  tube  is  charged  with  copper  oxide  as 
described  on  p.  322,  and  a  copper  roll,  reduced  with 
methyl  alcohol,  is  placed  in  the  front  end  of  the  tube 
(p.  328).  There  is  no  need  to  protect  the  copper  oxide 


330 


ANALYSIS  OF  ORGANIC  SUBSTANCES 


from  atmospheric  moisture,  but  it  must  be  ignited  in  order 

to  destroy  organic  matter. 

(2)  A  Jena  Glass  Test-tube,  1 5  cm.  x  2  cm.     This  tube  is 

nearly  filled  with  sodium  bicarbonate  (free  from  ammonia), 

a    plug    of    glass    wool    is    inserted,    and    the    tube,    held 

horizontally,  is  tapped  in  order 
to  form  an  air  space  above  the 
substance.  The  test-tube  is  held 
in  a  clamp,  and  is  connected  with 
the  rear  end  of  the  combustion- 
tube  by  means  of  a  V-shaped 
bulb-tube  (Fig.  85)  containing  a 
globule  of  mercury.  The  mercury 
acts  as  an  indicator  of  the  rate  at 
which  the  carbon  dioxide  passes 
into  the  combustion  tube  when 
the  sodium  bicarbonate  is  heated. 
Over  the  test-tube  is  slipped  a 
cylinder  of  wire  gauze,  which  serves 
to  distribute  the  heat  somewhat, 
and  prevents  water  condensing  and 
cracking  the  hot  glass. 


Copper  Coil 


Coarse 
Copper  Oxide 


Fine  Copper 
Oxide    and 
Substance 


Sodium 
Bicarbonate 


FIG.  86. 


(3)  A  Nitrometer,  Fig.  86.     Mercury  is  poured  into  the 
nitrometer  until  it  stands  about  5  mm.  above  the  lower  side- 


NITROGEN  BY  DUMAS'  METHOD  331 

tube.  The  reservoir,  clamped  in  its  lowest  position,  is  nearly 
filled  with  a  50  per  cent,  potassium  hydroxide  solution. 
In  using  the  nitrometer,  care  must  be  taken  that  the  caustic 
potash  solution  is  not  allowed  to  pass  the  mercury  seal  and 
to  enter  the  side-tube  that  connects  the  nitrometer  with  the 
combustion  tube ;  when  the  graduated  tube  is  filled  with  the 
solution  (by  opening  the  tap  and  raising  the  reservoir),  there 
must  be  sufficient  mercury  to  prevent  this.  The  rubber  tube 
connecting  the  reservoir  with  the  graduated  tube  must  be 
wired  on  at  each  end. 

Procedure. — If  the  substance  is  a  solid,  it  is  weighed  in 
the  tube  B  (Fig.  81)  and,  after  being  mixed  with  copper 
oxide,  is  transferred  to  the  combustion  tube  in  the  manner 
described  on  p.  324.  The  amount  of  substance  taken 
should  be  sufficient  to  yield  25  to  30  c.c.  of  nitrogen  at  the 
ordinary  temperature,  and  it  is  useful  to  bear  in  mind  that 
the  volume  of  I  milligram-atom  (0-014  gram)  of  nitrogen  at 
normal  temperature  and  pressure  is  1 1 -2  c.c.  The  combus- 
tion tube  is  then  laid  in  the  furnace,  and  the  rear  end  is 
connected  with  the  sodium  bicarbonate  tube  by  means  of  the 
V-tube  and  accurately  fitting  rubber  stoppers. 

Before  attaching  the  nitrometer,  most  of  the  air  in  the 
combustion  tube  is  displaced  by  a  fairly  rapid  current  of 
carbon  dioxide,  obtained  by  heating  the  sodium  bicarbonate 
tube  with  a  small  Bunsen  flame.  The  flame  is  applied  first 
at  the  closed  end  of  the  test-tube,  and  is  gradually  moved 
forward  as  the  current  of  gas  slackens.  After  the  carbon 
dioxide  has  been  passed  for  about  five  minutes,  the  burners 
under  the  front  portion  of  the  combustion  tube  are  lighted, 
covers  are  placed  over  this  part  of  the  tube,  and  the  nitro- 
meter, with  the  tap  open  and  the  reservoir  in  the  lowest 
position,  is  attached.  The  bent  connecting  tube  is  fixed  into 
the  combustion  tube  by  means  of  an  accurately  fitting  rubber 
stopper,  and  is  attached  to  the  side-tube  of  the  nitrometer  by 
means  of  a  short  piece  of  pressure  tubing,  which  can  be 
closed  with  a  screw-clip. 

In  order  to  determine  whether  the  air  in  the  tube  has 
been  completely  displaced,  the  potash  reservoir  is  slowly 
raised  and,  when  the  nitrometer  is  full  of  solution,  the  tap  is 
closed  and  the  reservoir  lowered  again.  It  is  practically 


332  ANALYSIS  OF  ORGANIC  SUBSTANCES 

impossible  to  secure  complete  absence  of  residual  air,  but  the 
bubbles  which  collect  at  the  top  of  the  nitrometer  ought  to 
be  so  minute  that  they  appear  as  a  foam  or  froth  of  in- 
appreciable volume.  If  this  is  not  the  case,  the  nitrometer  tap 
is  opened,  and  the  potash  is  run  back  into  the  reservoir, 
carbon  dioxide  is  passed  for  several  minutes  more,  and  the 
test  repeated. 

When  the  result  is  satisfactory,  the  flame  under  the 
bicarbonate  tube  is  lowered  until  the  current  of  carbon 
dioxide  is  very  slow,  the  nitrometer  is  filled  with  the  potash 
solution,  the  tap  closed,  and  the  reservoir  lowered  as  far  as 
possible.  When  the  front  portion  of  the  combustion  tube 
has  attained  a  dull  red  heat,  the  burner  under  the  rear  end 
of  the  tube  is  lighted,  and  the  mixture  of  copper  oxide 
and  substance  is  gradually  heated  in  the  same  way  as  in  a 
carbon  and  hydrogen  combustion.  The  heating  must  be  so 
regulated  that  the  bubbles  of  nitrogen  can  be  counted  as 
they  pass  into  the  nitrometer.  When  the  whole  rear  part  of 
the  tube  has  been  heated,  all  the  covers  are  laid  in  position, 
and  the  flame  spreader  is  placed  on  the  rear  burner.  When 
the  evolution  of  gas  becomes  very  slow,  the  residual  nitrogen 
is  expelled  from  the  tube  by  a  fairly  rapid  current  of  carbon 
dioxide  (obtained  by  again  heating  the  sodium  bicarbonate). 

Finally,  when  the  bubbles  of  gas  appear  to  be  completely 
absorbed  and  the  volume  of  nitrogen  no  longer  increases, 
the  nitrometer  is  detached  by  removing  the  cork  from  the 
combustion  tube,  and  the  screw-clip  is  closed.  The  reservoir 
is  then  raised  until  the  surface  of  the  liquid  it  contains  is 
approximately  level  with  that  in  the  measuring-tube,  and 
the  nitrometer,  with  a  thermometer  hanging  from  the  tap,  is 
left  for  an  hour  in  the  balance-room,  or  other  cool  place. 

Before  stopping  the  current  of  carbon  dioxide,  the  front 
portion  of  the  combustion  tube  is  allowed  to  cool  and  the 
copper  roll  is  removed.  A  current  of  oxygen  is  then  passed 
through  the  heated  tube  in  order  to  oxidise  the  reduced 
copper  oxide.  After  the  tube  is  cold,  the  oxide  in  the 
rear  portion  is  transferred  to  the  tube  A  (Fig.  81,  on  p.  322). 

The  volume  of  nitrogen  obtained  (v)  is  then  measured, 
after  carefully  equalising  the  levels  of  the  liquid  surfaces  in 
the  reservoir  and  measuring-tube,  and  the  temperature  (/) 


NITROGEN  BY  DUMAS'  METHOD 


333 


and  barometric  pressure  (£)  are  noted.  The  vapour  pressure 
(/)  of  the  potash  solution  at  t°  will  be  found  in  Table  on 
p.  371.  The  volume  (  F)  of  the  nitrogen  at  normal  tem- 
perature and  pressure  is  then 


760  x  (273  +  0' 

and,  since  I  c.c.  of  nitrogen  at  N.T.P.  weighs  0-001250  gram, 
the  percentage  of  nitrogen  in  the  substance,  if  w  is  the 
weight  taken  is 


w 

Instead  of  measuring  the  nitrogen  over  the  potash 
solution  —  the  vapour  pressure  of  which,  after  partial 
conversion  into  potassium  carbonate,  is 
somewhat  uncertain  —  the  gas  may  be 
transferred  to  a  graduated  tube  filled 
with  water,  and  standing  over  water 
contained  in  a  tall  cylinder.  To  accom- 
plish this,  the  cup  of  the  nitrometer  is 
filled  with  water  and  a  bent  delivery 
tube,  also  filled  with  water,  is  attached 
by  means  of  a  rubber  stopper  (Fig.  87). 
No  air  must  be  present  in  the  cup  or 
delivery  tube.  The  graduated  tube  is 
brought  over  the  end  of  the  delivery 
tube  and  clamped  in  position.  On  now 
raising  the  potash  reservoir  and  opening 
the  tap,  the  gas  passes  over  into  the  '  7' 

graduated  tube.  The  tube  is  then  immersed  in  the  water 
for  several  minutes,  after  which  the  volume  of  the  gas  at 
atmospheric  pressure  is  read.  While  adjusting  the  level 
of  the  water  surfaces,  the  tube  is  held  in  a  collar  of  paper. 
The  temperature  of  the  gas  is  the  temperature  of  the  water, 
and  the  pressure  is  equal  to  the  barometric  pressure  minus 
the  vapour  pressure  of  water  (Table  on  p.  371)  at  the  tem- 
perature of  observation. 

The  transference  of  the  nitrogen  to  the  graduated  tube  is 
greatly  facilitated  if  a  nitrometer  of  the  type  shown  in  Fig.  86 


334  ANALYSIS  OF  ORGANIC  SUBSTANCES 

is  used.  The  cup  is  filled  with  water ;  the  graduated  tube, 
filled  with  water,  is  placed  over  the  small  inner  tube ;  and  the 
nitrogen  is  driven  into  the  graduated  tube.  The  mouth  of 
the  tube  is  closed  with  the  thumb,  the  tube  transferred  to  a 
tall  cylinder  filled  with  water,  and  the  nitrogen  is  measured 
as  already  described. 

Nitrogen  by  Kjeldahl's  Method. 

OUTLINE  OF  METHOD. — The  substance  is  decomposed  by  prolonged 
boiling  with  concentrated  sulphuric  acid,  whereby  the  nitrogen  is 
converted  quantitatively  into  ammonium  sulphate.  The  ammonia  is 
then  determined  by  one  of  the  usual  methods. 

Procedure. — Place  0-5  to  I  gram  of  the  substance,  e.g., 
acetanilide,  in  a  weighing-bottle,  weigh  accurately,  empty 
it  into  a  200  c.c.  Kjeldahl  flask,  and  weigh  again.  The 
difference  represents  the  weight  taken  for  the  analysis.  A 
Kjeldahl  flask  (A)  is  a  round-bottomed,  hard-glass  flask  with 

a  long  narrow  neck ;  if  this 
is  not  available,  an  ordinary 
round-bottomed  flask  of  about 
500  c.c.  capacity  may  be  used. 
A  loose  stopper  for  the  flask 
is  made  by  blowing  a  bulb 
on  a  piece  of  narrow  tubing, 
as  shown  in  Fig.  B. 

Support    the  flask    in    an 
inclined  position  on  a  piece 

Kir    no 

of  asbestos  board  C,  in  which 
a  hole  about  2  inches  in  diameter  has  been  cut. 

Add  20  c.c.  of  concentrated  sulphuric  acid  and  10  grams 
of  potassium  sulphate  and  heat  until  nearly  boiling.  Addi- 
tion of  potassium  sulphate  raises  the  boiling-point,  and  the 
reaction  proceeds  faster  than  when  sulphuric  acid  alone  is  used. 

Keep  the  solution  near  the  boiling-point  until  it  has 
become  colourless.  (If  there  has  not  been  any  charring, 
heat  fof  one  hour.) 

Cool,  and  dilute  with  about  100  c.c.  of  water.  Pour  the 
solution  and  washings  into  the  copper  flask  of  the  apparatus 
described  on  p.  59.  Dissolve  about  35  grams  (three  "white 
sticks ")  of  sodium  hydroxide  in  water,  run  this  in  through 


NITROGEN  BY  KJELDAHL'S  METHOD  335 

the  tap-funnel,  and  boil  until  all  the  ammonia  is  expelled. 
The  ammonia  is  absorbed  by  a  measured  volume  of  standard 
acid  and  the  excess  of  acid  found  by  titration  with  standard 
alkali,  as  already  described  under  the  direct  method  for  the 
determination  of  ammonia. 

Example. — The  ammonia  obtained  from  0-651  gram  of 
urea  was  absorbed  by  25-0  c.c.  of  N  sulphuric  acid.  3-40  c.c. 
of  N  sodium  hydroxide  was  required  to  neutralise  the  un- 
used acid.  The  ammonia  has  therefore  neutralised  21-60  c.c. 
of  N  acid.  Each  cubic  centimetre  of  N  acid  neutralises 
0-01703  gram  of  NH3,  which  is  equivalent  to  0-01401  gram 
of  nitrogen. 

The  substance  therefore  contained  21-60x0-01401  gram 
nitrogen — i.e.,  0-651  gram  contained  0-3026  gram  nitrogen 
=  46-5  per  cent. 

Notes. — Most  substances  are  decomposed  in  a  reasonable 
time  by  a  mixture  of  sulphuric  acid  and  potassium  sulphate. 
If  the  decomposition  is  very  slow,  the  reaction  may  be 
hastened  by  addition  of  about  o-i  gram  of  mercury,  copper 
sulphate,  or  manganese  dioxide.  Since  mercury  forms 
complex  salts  with  ammonium  salts,  and  these  do  not  yield 
ammonia  readily  with  sodium  hydroxide,  it  is  necessary  to 
add  with  the  sodium  hydroxide  a  little  sodium  sulphide 
to  precipitate  the  mercury.  The  presence  of  the  sulphide 
precipitate  has  no  disturbing  effect. 

Commercial  sulphuric  acid  sometimes  contains  nitrogen 
compounds  as  impurities.  The  presence  of  nitrogen  may  be 
detected  by  boiling  with  aluminium  and  excess  of  sodium 
hydroxide  when  any  nitrogen  will  be  expelled  as  ammonia. 
(Caution. — Dilute  the  sulphuric  acid  before  mixing  with  the 
sodium  hydroxide.)  If  nitrogen  is  present  the  amount  must 
be  determined  quantitatively.  Use  20  c.c.  of  the  concen- 
trated acid  and  proceed  as  directed  under  "  Nitrate,"  on  p.  60. 
This  will  give  the  total  nitrogen  in  whatever  form  it  may  be. 

CHLORINE,  BROMINE,  AND  IODINE. 

In  all  the  methods  for  the  determination  of  the  halogens 
in  organic  compounds,  there  is  one  common  feature,  viz.,  the 
compound  is  decomposed  in  such  a  way  that  the  amount  of 


336  ANALYSIS  OF  ORGANIC  SUBSTANCES 

the  halogen  can  be  ascertained  either  gravimetrically  by 
weighing  the  corresponding  silver  halide,  or  volumetrically 
by  means  of  standard  solutions  of  silver  nitrate  and 
ammonium  thiocyanate.  The  methods  differ  in  the  manner 
in  which  the  decomposition  of  the  substance  is  effected. 

In  Stepanow's  method,  the  substance  is  decomposed  by 
means  of  sodium  in  presence  of  alcohol.  The  reaction  may 
be  represented  by  the  equation — 

RC1  +  2Na  +  C2H5OH  =  RH  +  NaCl  +  C2H5ONa. 

The  halogen,  which  is  thus  obtained  as  a  sodium  salt,  is  then 
determined  volumetrically.  A  much  larger  quantity  of  sodium 
than  the  equation  indicates  must  be  used,  and  the  amount 
required  varies  with  the  nature  of  the  halogen.  Whatever 
the  nature  of  the  halogen  may  be,  take  about  02  gram  of 
the  substance.  For  chlorine,  use  4  grams  of  sodium  and 
30  c.c.  of  98  per  cent,  alcohol ;  for  bromine,  use  one-half  of 
these  quantities;  and  for  iodine,  use  1*5  grams  of  sodium 
and  about  12  c.c.  of  alcohol. 

Procedure. — Weigh  accurately  about  02  gram  of  the 
substance,  and  place  it,  together  with  the  alcohol,  in  a  dry 
flask  (200  c.c.)  fitted  with  a  reflux  condenser.  Warm  the 
mixture  on  the  steam  -  bath,  and  introduce  the  sodium 
through  the  condenser  at  a  rate  sufficient  to  maintain  a 
vigorous  reaction.  (The  addition  of  the  sodium  should 
occupy  about  half  an  hour.)  Boil  the  mixture  for  one  hour, 
cool,  and  add  about  30  c.c.  of  water  through  the  condenser. 
Acidify  the  solution  with  nitric  acid,  and  determine  the 
halogen  by  means  of  decinormal  silver  nitrate  and  thio- 
cyanate (p.  103). 

The  method  gives  satisfactory  results,  even  when  the 
halogen  is  directly  attached  to  a  benzene  nucleus. 

SULPHUR. 

The  amount  of  sulphur  in  an  organic  compound  is 
ascertained  by  oxidising  the  substance  in  such  a  manner  as 
to  convert  all  the  sulphur  into  sulphate.  The  sulphate  is 
then  determined  gravimetrically  as  barium  sulphate. 

Except  with  volatile  substances,   this   oxidation  is  con- 


SULPHUR  IN  AN  ORGANIC  COMPOUND  337 

veniently   effected   by   heating  the   substance  with   sodium 
peroxide  and  sodium  carbonate  as  described  below. 

Procedure. — In  a  nickel  crucible  mix  a  weighed  portion 
(from  0-2  to  0-5  gram)  with  10  grams  of  anhydrous  sodium 
carbonate  (free  from  sulphate).  When  these  are  thoroughly 
mixed,  add  5  grams  of  sodium  peroxide,  and  stir  until  com- 
pletely mixed.  Heat  with  a  small  flame,  held  several  inches 
from  the  crucible.  When  the  mixture  shrinks  together  and 
begins  to  melt,  raise  the  temperature  gradually  until  the 
mixture  forms  a  clear  thin  liquid. 

Cool,  place  the  crucible  and  contents  in  a  beaker,  and 
cover  with  water.  Add  bromine  water  until  the  solution  is 
coloured  with  bromine,  and  warm  on  the  steam-bath  for 
thirty  minutes.  Remove  the  nickel  crucible  and  rinse  it 
thoroughly  with  hot  water. 

Filter,  and  wash  the  residue  with  hot  water.  If  the  first 
portion  of  the  filtrate  is  not  colourless,  return  it  to  the 
beaker,  boil  for  a  minute  after  addition  of  a  little  magnesium 
oxide,  and  again  filter. 

Acidify  the  solution  with  hydrochloric  acid  and  evaporate 
to  dryness,  in  order  to  render  the  silica  which  is  usually 
present  insoluble  (cf.  p.  206). 

After  removal  of  the  silica,  determine  the  sulphate  as 
described  on  p.  131. 


PART   X 

THE     DETERMINATION     OF 
MOLECULAR   WEIGHTS 

IT  is  often  necessary,  more  particularly  in  organic  analysis, 
to  determine  which  multiple  of  the  empirical  formula  of  a 
substance  represents  its  molecular  formula.  Obviously,  a 
rough  approximation  to  the  true  molecular  weight  is 
sufficient  for  this  purpose. 

Whenever  possible,  the  molecular  weight  should  be 
determined  by  two  or  more  methods,  since  the  molecular 
complexity  of  most  substances  is  not  the  same  under  all 
conditions.  To  illustrate  the  danger  of  relying  on  one 
method,  the  case  of  benzoic  acid  in  benzene  solution  may 
be  cited.  The  molecular  weight  of  benzoic  acid  in  benzene 
solution  is  about  240,  and  its  formula  is  therefore 
(C6H5COOH)2;  in  most  other  solvents,  however,  the 
molecular  weight  is  about  122,  in  agreement  with  the 
simpler  formula,  C6H5COOH.  This  does  not  mean  that 
the  result  in  benzene  solution  is  wrong,  but  that  it  represents 
an  unusual  condition  of  the  substance. 

The  chief  methods  in  common  use  for  determining  molec- 
ular weights  are  based  on  the  determination  of  (i)  the  vapour 
density  ;  (2)  the  freezing-point  of  a  solution  of  the  substance  ; 
and  (3)  the  boiling-point  of  a  solution  of  the  substance.  The 
vapour  density  method  has  the  advantage  over  the  other 
methods  that  no  solvent  is  used,  and  it  gives  the  molec- 
ular weight  of  a  substance  in  the  gaseous  state;  the 
other  methods  give  the  molecular  weight  of  the  dissolved 
substance. 


VAPOUR  DENSITY  339 

DETERMINATION  OP  VAPOUR  DENSITY 
AND  MOLECULAR  WEIGHT. 

The  molecular  weight  of  a  gas  is  that  weight  of  the  gas, 
in  grams,  which  occupies  22-4  litres  at  o°  and  760  mm. 
Provided  a  substance  can  be  converted  into  vapour,  the 
molecular  weight  of  its  vapour  can  be  ascertained  by 
measuring  its  vapour  density.  The  practical  problem 
resolves  itself  into  finding  the  volume,  temperature,  and 
pressure  of  a  known  weight  of  the  substance  in  the  state  of 
gas.  This  can  be  done  most  conveniently,  and  with  an 
accuracy  sufficient  for  ordinary  purposes,  by  either  Victor 
Meyer's  (constant  pressure)  or  Lumsden's  (constant  volume) 
method. 

CONSTANT  PRESSURE  METHOD. 

OUTLINE  OF  METHOD.— A  known  weight  of  the  substance  is  converted 
into  vapour  in  a  suitable  apparatus,  and,  the  pressure  being  constant 
(equal  to  that  of  the  atmosphere),  the  increase  in  volume,  due  to  the 
formation  of  the  vapour,  is  determined.  In  other  words,  the  volume 
of  a  known  weight  of  the  substance,  measured  under  definite 
conditions  of  temperature  and  pressure,  is  ascertained.  It  is  then 
easy  to  calculate  what  weight  of  the  substance  would  occupy,  in  the 
gaseous  state,  22-4  litres  at  normal  temperature  and  pressure,  *.*., 
its  molecular  weight. 

The  Apparatus  (Fig.  89)  consists  essentially  of  a  tube  A 
in  which  the  substance  is  vaporised,  and  a  burette  B  in 
which  the  air  displaced  by  the  vapour  of  the  substance 
is  measured.  The  tube  A  is  provided  with  two  side-tubes 
C  and  D.  C  is  connected  with  the  burette  by  means  of  a 
glass  tube  about  18  inches  long.  The  tap  H  is  convenient, 
but  is  not  essential.  A  glass  rod  passes  through  the  side- 
tube  D,  the  joint  being  made  air-tight  by  means  of  a  piece 
of  rubber  tubing  wired  on  over  both  rod  and  tube.  The  open 
end  of  A  is  closed  with  a  rubber  stopper  E.  The  burette 
is  connected  with  a  levelling-tube  fitted  with  a  jet  and 
spring  clip. 

The  bulb  A  must  be  heated  to  a  temperature  at  least 
25°  higher  than  the  boiling-point  of  the  substance  under 
investigation.  If  the  boiling-point  of  the  substance  is 


340     DETERMINATION  OF  MOLECULAR  WEIGHTS 


B 


below  75°,  the  heating  is  most  conveniently  performed  by 
means   of    steam,  using   the  jacket    shown   in    Fig.  91,  on 

_  F  P-  343- 

If  the  substance  boils  at  a  tem- 
perature above  75°,  steam  cannot 
be  used  as  a  source  of  heat.  The 
apparatus  shown  in  Fig.  89  must 
then  be  used.  A  suitable  liquid 
is  boiled  in  the  tube  G  with  such 
vigour  that  its  vapour  nearly  fills  the 
tube.  Aniline  (boiling-point  183°), 
nitrobenzene  (boiling-point  205°), 
quinoline  (boiling-point  236°),  and 
a  -  bromnaphthalene  (boiling-point 
279°)  are  useful  heating  liquids. 
For  still  higher  temperatures,  the 
apparatus  must  be  made  of  Jena 
glass  or  of  silica.  Sulphur  (boiling- 
point  445°)  or  other  substances  of 
high  boiling-point  may  be  used, 
but  it  is  usually  preferable,  for 
very  high  temperatures,  to  use  a 
bath  of  molten  metal,  e.g.,  tin  (melting-point  232°).  In  this 
case  the  bulb  must  be  completely  immersed  in  the  heating 
liquid  and  the  temperature  must  be  kept  constant,  although, 
so  long  as  it  is  high  enough,  it  is  not  necessary 
to  know  what  the  temperature  is. 

In  order  to  prevent  the  tube  A  from  touch- 
ing the  wall  of  the  outer  vessel,  two  pieces  of 
string  may  be  tied  round  the  bulb. 

The  substance  is  weighed  in  a  narrow  tube, 
about  I  inch  long,  of  thin  glass  (Fig.  90).  This 
tube  is  not  stoppered,  but  is  provided  with  a 
cap,  which  is  removed  just  before  the  tube  is 
dropped  into  the  apparatus.  With  a  little  care 
this  arrangement  is  quite  satisfactory  even  with 
a  very  volatile  liquid  such  as  ether.  It  is 
convenient  to  fix  the  tube  into  a  piece  of  cork 
weighing. 

In  order  to  protect  the  bulb  A  from  fracture  when  the 


J 

HP 


FIG.  89. 


FIG  90. 

Weighing-tube 

and  cap  (full 

size). 


during 


VICTOR  MEYER'S  METHOD  341 

weighing-tube  is  dropped  into  it,  a  quantity  of  mercury 
should  be  placed  in  the  bulb ;  the  mercury  also  greatly 
accelerates  the  rate  of  vaporisation  of  the  substance.  At 
temperatures  above  150°,  fusible  metal,  or  a  small  pad  of 
asbestos  fibre,  should  be  used  instead  of  mercury.  The 
upper  part  of  the  tube  A  is  protected  from  the  heat  by  means 
of  a  piece  of  asbestos  board  resting  on  the  top  of  the  tube  G 
(Fig.  89). 

Procedure. — Clean  the  tube  A  and  dry  it  carefully  by 
warming  and  blowing  a  current  of  air  into  it.  Pour  into  the 
tube  about  10  c.c.  of  dry  mercury  and  place  it  inside  the 
vapour  jacket.  Almost  fill  the  burette  and  levelling-tube 
with  water,  and  connect  the  burette  with  A.  Hang  a 
thermometer  close  to  the  burette.  Insert  the  cork  E,  open 
the  tap  H,  and  boil  the  liquid  in  G  (or  pass  a  fairly  rapid 
current  of  steam  if  the  steam  jacket  shown  in  Fig.  91  is 
used). 

Meantime  weigh  accurately  a  suitable  quantity  (e.g.>  about 
0-08  gram  acetone)  of  the  substance  in  the  small  tube.  Then 
close  the  tap  H,  and  note  whether  the  level  of  the  water  in 
the  burette  remains  unchanged  during  the  next  minute  or 
so.  If  not,  open  H  again,  and  after  a  few  minutes  repeat 
the  test.  When  constant  temperature  is  attained,  open  the 
tap  H,  remove  the  stopper  E,  and  carefully  drop  the 
weighing-tube,  without  the  cap,  on  to  the  rod  at  D.  Replace 
the  stopper  E,  close  H,  and  read  the  burette.  Then,  by 
moving  the  rod  D,  allow  the  tube  containing  the  substance 
to  drop  to  the  bottom  of  A.  The  liquid  quickly  vaporises. 
As  the  air  which  is  expelled  from  A  passes  over  into  the 
burette,  run  ofT  water  at  the  jet  so  as  to  keep  the  levels  of 
the  water  in  the  burette  and  levelling-tube  about  the  same ; 
error  through  leakage  is  thus  minimised.  In  about  a  minute 
or  less,  expansion  ceases  and  the  volume  of  air  in  the  burette 
becomes  constant.  Now  equalise  the  water  levels,  in  order 
to  bring  the  air  in  the  burette  under  atmospheric  pressure, 
and  read  the  burette.  Note  the  temperature  and  ascertain 
the  height  of  the  barometer. 

The  difference  between  the  burette  readings  gives  the 
volume  of  air  equal  to  the  volume  of  the  vapour  measured 
at  room  temperature  and  atmospheric  pressure.  Calculate 


342     DETERMINATION  OF  MOLECULAR  WEIGHTS 

what  this  volume  would  be  at  o°  and  760  mm.,  and  then 
find  what  weight  of  the  substance,  if  it  were  a  gas,  would 
occupy  22-4  litres  at  normal  temperature  and  pressure.  This 
is  the  molecular  weight  of  the  substance. 

Precautions  and  Notes. — The  substance  must  vaporise 
quickly,  otherwise  part  of  the  vapour  may,  by  diffusion, 
reach  the  upper  and  colder  portion  of  the  tube  and  condense 
there.  Since  the  air  in  the  burette  is  measured  over  water, 
its  pressure  is  equal  to  the  barometric  pressure  minus  the 
vapour  pressure  of  water  at  the  room  temperature.  Consult 
the  table  of  vapour  pressures  of  water  for  the  correction 

(P-  3/1). 

At  the  conclusion  of  an  experiment,  a  glass  tube  is  passed 

down  into  the  bulb  A,  and  the  vapour  is  removed  by  means 
of  the  water-pump.  Another  weighing-tube,  containing  a 
fresh  portion  of  the  substance,  is  then  dropped  in  as  before. 

CONSTANT   VOLUME    METHOD. 

OUTLINE  OF  METHOD. — A  known  weight  of  the  substance  is  vaporised 
in  an  apparatus  of  constant  volume,  and  the  increase  of  pressure, 
due  to  the  formation  of  the  vapour,  is  measured.  If  the  temperature 
of  the  vapour  and  the  volume  of  the  apparatus  are  known,  the 
molecular  weight  of  the  vapour  can  be  calculated. 

The  Apparatus  (Fig.  91)  is  similar  to  that  used  in 
Victor  Meyer's  method,  but  the  tube  A  may  be  much 
shorter,  and  a  manometer  M  takes  the  place  of  the  burette. 
The  manometer  is  graduated  in  millimetres,  and  contains 
sufficient  mercury  to  fill  it  almost  to  the  top  of  the  graduated 
portion  when  the  meniscus  in  the  other  limb  is  at  a  fixed 
mark  B.  The  position  of  this  mark  determines  the  volume 
of  the  vaporisation  tube.  In  order  to  heat  the  bulb  A,  either 
a  closed  tube  (Fig.  89)  or  a  steam  jacket1  (Fig.  91)  is  used. 
When  using  a  closed  tube,  a  groove  should  be  cut  in  the  cork 
to  allow  the  air  in  the  tube  to  expand,  or  a  condenser  may 
be  added  if  necessary.  The  volume  of  the  vaporisation  tube 

1  The  steam  used  should  be  dry.  The  water-trap  shown  at  the  side 
of  the  diagram  is  simple  and  efficient.  The  clip  on  the  waste  pipe  is 
opened  sufficiently  to  run  off  the  water  without  allowing  much  steam  to 
escape. 


LUMSDEN'S  METHOD 


343 


is  found  by  weighing  the  tube  empty  and  then  full  of  water. 
It  is  unnecessary,  however,  to  know  the  volume  of  the  tube 
or  the  temperature  of  the  heating  jacket — provided  these 
are  maintained  constant  during  the  experiment — and  the 
following  method  of  procedure  is  convenient,  especially  for 


K 


FIG.  91. 

high  temperatures,  when  it  is  easier  to  keep  the  temperature 
constant  for  a  time  than  to  determine  exactly  what  the 
temperature  is. 

First  determine  the  increase  of  pressure  produced  by 
vaporising  a  known  weight  of  a  substance  of  known 
molecular  weight,  and  calculate  from  this  the  increase  of 
pressure  that  would  be  observed  if  one  gram-molecular 


344     DETERMINATION  OF  MOLECULAR  WEIGHTS 

weight  of  the  substance  were  used.  In  the  same  apparatus 
(i.e.,  in  the  same  volume)  and  at  the  same  temperature  of 
vaporisation,  one  gram-molecular  weight  of  any  substance 
would  give  the  same  increase  of  pressure — a  constant  which 
may  be  regarded  as  the  molecular  increase  of  pressure  for 
the  apparatus.  (Cf.  the  "  molecular  elevation  "  of  the  boiling- 
point.)  Then  determine  the  increase  of  pressure  produced 
by  vaporising  a  known  weight  of  the  substance  under 
investigation,  and  from  this  calculate  what  weight  of  the 
substance  would  give  a  pressure  equal  to  that  produced  by 
one  gram-molecular  weight  of  the  known  substance,  i.e., 
equal  to  the  constant  molecular  increase  of  pressure  for  the 
apparatus.  If  w  grams  of  a  substance  produce  a  rise  of 
pressure  /,  the  molecular  weight  of  the  substance  is  equal 

to  K— ,  where  K  is  the  "  molecular  increase." 
P 

Procedure. — Clean  and  dry  the  tube  A  and  pour  into  it 
about  15  c.c.  of  mercury.  Fill  the  manometer  with  mercury, 
fit  the  apparatus  together,  and  start  the  preliminary  heating 
— the  tap  H  being  open.  Clamp  the  manometer  in  such  a 
position  that  the  mercury  stands  at  the  mark  B.  When 
temperature  equilibrium  is  attained,  the  mercury  will  remain 
at  the  mark  when  the  tap  H  is  closed. 

Weigh  accurately  in  the  capped  tube  (Fig.  90)  a  quantity 
of  a  substance  (of  known  molecular  weight)  sufficient  to  give 
a  rise  of  pressure  of  100  to  200  mm.  (e.g.,  about  0-08  gram 
acetone  if  the  volume  of  the  tube  A  is  about  200  c.c.).  Drop 
the  tube  (without  the  cap)  on  to  the  rod  at  D,  replace  the 
stopper  E,  and  close  the  tap  H.  Let  the  tube  and  contents 
fall  into  the  bulb,  and,  as  vaporisation  proceeds,  slowly  raise 
the  tube  M  in  order  to  keep  the  mercury  near  the  mark  B. 
In  less  than  a  minute  the  vaporisation  is  complete  and  the 
mercury  becomes  stationary.  Now  place  the  manometer 
tubes  close  together,  and  carefully  adjust  and  clamp  the 
tube  M  so  that  the  mercury  stands  at  the  mark  B.  Read 
off  the  position  of  the  mercury  in  the  tube  M,  and  also  note 
the  graduation  at  the  level  of  the  mark  B,  i.e.,  find  the 
distance  in  millimetres  between  the  two  mercury  surfaces. 
This  is  the  increase  of  pressure  produced  with  a  known 
weight  of  the  substance,  and  it  is  easy  to  calculate  the 


FREEZING-POINT  METHOD  345 

increase  that  would  have  been  observed  if  one  gram-molecular 
weight  had  been  used. 

Now  cautiously  open  the  tap  H  and  at  the  same  time 
lower  the  tube  M,  and  remove  the  vapour  from  the  apparatus 
by  means  of  a  current  of  air.  Repeat  the  experiment  with 
the  same  substance  several  times,  and  take  the  mean  of  the 
results  as  the  "molecular  increase." 

Having  in  this  way  "  standardised  "  the  apparatus, 
determine  the  increase  of  pressure  produced  by  a  known 
weight  of,  for  example,  methyl  iodide,  methyl  alcohol, 
chloroform,  or  ether,  and  calculate  the  molecular  weight  of 
the  substance. 

THE   FREEZING-POINT   METHOD. 

The  freezing-point  of  a  solution  is  always  lower  than  that 
of  the  pure  solvent  and,  with  dilute  solutions,  the  depression 
of  the  freezing-point  is  proportional  to  the  molecular 
concentration  of  the  dissolved  substance. 

If  s  grams  of  a  substance  are  dissolved  in  w  grams  of  a 
solvent,  and  lower  the  freezing-point  of  the  solvent  A°,  the 
molecular  weight,  M,  of  the  substance  may  be  calculated 
from  the  formula, 


where  K  is  a  constant  depending  on  the  nature  of  the 
solvent.  K  is  the  amount  by  which  the  freezing-point  of 
the  solvent  would  be  lowered  by  dissolving  i  gram- 
molecule  of  a  substance  in  i  gram  of  the  solvent.  The 
value  of  K  for  any  particular  solvent  can  be  calculated  by 
simple  proportion  from  the  observed  depression  of  the 
freezing-point,  produced  by  dissolving  a  known  weight  of  a 
substance  of  known  molecular  weight  in  a  known  quantity 
of  the  solvent  ;  it  may  also  be  calculated  from  the  latent 
heat  of  fusion  of  the  solvent.  The  constants  for  the 
commonest  solvents  are  given  in  the  following  table  :  — 

Solvent.  Freezing-point.          K. 

Acetic  acid        .        .        .         17°  3,880 
Benzene    ....          5-5°  5,000 

Bromoform        .       '.         .          7.5°  14,400 
Water        ....          o°  1,870 


346     DETERMINATION  OF  MOLECULAR  WEIGHTS 

It  is  often  convenient  to  use  a  known  volume  of  the  solvent 
instead  of  a  known  weight.  The  molecular  weight  can  then 
be  calculated  from  the  formula, 


where  v  is   the  volume  in   cubic  centimetres  at    15°  of  the 
solvent.     The  values  for  k  are  as  follows  :  — 

Acetic  acid  ,  .  «  .  3,700 

Benzene    .  ,  ,  .  ,  5,650 

Bromoform  .  .  /  .  5,ioo 

Water       .'  .  .  .  .  1,870 

These  formulae  hold  true  only  when  the  solvent  separates 
in  a  pure  state  ;  the  freezing-point  method  for  determining 
molecular  weights  cannot  be  used,  therefore,  if  the  solvent 
and  solute  form  mixed  crystals. 

The  Thermometer.  —  The  above  formula  for  the  calcula- 
tion of  molecular  weights  is  applicable  only  to  experiments 
with  dilute  solutions,  with  which  the  observed  depression  of 
the  freezing-point  is  a  small  fraction  of  a  degree,  e.g.t  addition 
of  one-tenth  of  a  gram-molecule  of  a  substance  per  litre 
depresses  the  freezing-point  of  water  by  only  0-18°. 

For  most  purposes,  however,  it  is  sufficient  to  determine 
a  molecular  weight  accurate  to  within  5  per  cent.  The 
formula  can  be  applied,  with  about  this  degree  of  accuracy, 
to  experiments  with  solutions  up  to  0-5  gram-molecule 
per  litre,  corresponding  to  a  depression  of  0-9°  with  water 
and  2-5°  with  benzene.  For  this  type  of  work,  a  thermometer 
graduated  in  tenths  or  twentieths  of  a  degree,  and  with  an 
open  scale  which  can  be  read  to  hundredths,  is  sufficiently 
accurate. 

For  more  accurate  work,  a  Beckmann  thermometer  (see 
p.  351),  must  be  used. 

The  Apparatus  (Fig.  92)  consists  of  a  large  test-tube  A, 
provided  with  a  side  -tube  B,  and  placed  inside  a  wider 
tube  C,  so  as  to  be  surrounded  by  an  air-jacket.  The  wider 
tube  C  is  supported  by  the  metal  cover  of  the  cooling- 
bath  D,  and  it  is  convenient  to  weight  the  tube  with  shot 
so  that  it  will  not  float.  The  freezing-point  tube  A  is 
fitted  with  a  cork  carrying  the  thermometer  and  a  short 


FREEZING-POINT  METHOD 


347 


glass  tube  through  which  the  stirrer  S  can  pass  freely.     The 
stirrer  S    is   made   of  stout  nickel  wire  (or  of  glass  if  the 
liquid  attacks  nickel),  and  is  pro- 
vided with  a  cork  handle  to  pre- 
vent conduction  of  heat. 

The  temperature  of  the  cooling- 
bath  must  be  kept  from  3°  to  5° 
below  the  freezing-point  of  the 
solvent ;  an  ordinary  thermometer 
is  placed  in  the  cooling-bath,  and 
the  temperature  is  observed  from 
time  to  time  to  make  sure  that 
this  condition  is  observed.  For 
experiments  with  water  as  solvent, 
a  mixture  of  ice  and  concentrated 
brine  may  be  used ;  for  experi- 
ments with  benzene,  the  cooling- 
bath  may  be  kept  at  about  2°  by 
the  use  of  water  to  which  a  few 
lumps  of  ice  are  added  occasionally. 
The  level  of  the  liquid  in  the 
cooling-bath  should  be  about  an 
inch  higher  than  the  level  of  the 
liquid  in  the  freezing-point  tube. 
The  mixture  in  the  cooling-bath 
must  be  stirred  occasionally. 

Determination  of  the  Molecular 
Weight  of  a  Substance  in 
Benzene. 


Clean  and  dry  the  freezing- 
point  tube.  By  means  of  a  dry 
pipette  introduce  10  c.c.  of  pure 
benzene,  and  arrange  the  apparatus 
as  shown  in  Fig.  92.  The  dimen- 
sions of  the  freezing-point  tube 
should  be  such  that  with  10  c.c.  of 


FIG.  92. 


liquid  the  top  of  the  thermometer  bulb  is  at  least  J  mm. 
below  the  top  of  the  liquid,  whilst  the  bottom  of  the  bulb 


348     DETERMINATION  OF  MOLECULAR  WEIGHTS 

is   at   least   7  mm.  from  the   bottom    of  the    freezing-point 
tube. 

If  a  weight  measurement  of  the  solvent  is  desired,  attach 
a  piece  of  fine  wire  to  the  neck  of  the  tube  so  that  it  can 
be  suspended  from  the  hook  of  the  balance.  Weigh  the 
dry  tube,  and  weigh  again  after  the  addition  of  from  7  to 
10  grams  of  benzene.  It  is  sufficient  to  weigh  to  the  nearest 
centigram. 

First  determine  the  freezing-point  of  the  pure  solvent  as 
follows : — 

Place  the  freezing-point  tube  directly  in  the  cooling-bath, 
and  stir  the  benzene  steadily  until  the  temperature  falls  to 
about  6°.  Then  remove  the  tube  from  the  bath  and,  after 
drying  the  exterior,  place  it  at  once  within  the  tube  C. 
Stir  very  slowly  until  the  temperature  falls  to  about  5°,  and 
then  stir  vigorously  in  order  to  induce  crystallisation.1  As 
soon  as  the  freezing  commences,  the  temperature  rises 
quickly.  From  this  point  stir  steadily — but  not  vigorously — 
and  observe  the  temperature  as  accurately  as  possible  with 
the  aid  of  a  lens  until  it  becomes  constant.  In  order  to 
reduce  the  "  accidental "  error  of  experiment,  determine  the 
freezing-point  three  times,  and  take  the  mean  value.  In 
making  a  second  or  third  observation  of  the  freezing-point, 
remove  the  tube  with  the  thermometer  and  fittings  from  the 
wider  tube,  and  warm  it  with  the  hand  until  almost  the  whole 
of  the  crystals  have  melted.  Replace  the  tube  and  stir  the 
liquid.  The  remaining  crystals  melt  before  the  liquid 
begins  to  cool  again.  Proceed  as  in  the  first  measurement 
of  the  freezing-point. 

When  the  freezing-point  of  the  pure  solvent  has  been 
ascertained,  add  a  weighed  quantity  of  the  substance.  As  it 
is  desirable  to  determine  the  freezing-points  of  solutions  of 
different  concentrations,  place  about  i  gram  of  the  substance 
in  a  small  stoppered  weighing-tube,  and  weigh.  Empty  from 
0-05  to  oio  gram  of  the  substance  (sufficient  to  give  a 

1  The  solid  solvent  must  separate  in  fine  crystals.  The  formation  of 
solid  crusts  on  the  side  of  the  tube  may  be  caused  by  (i)  inefficient 
stirring ;  (2)  too  rapid  cooling,  either  from  the  bath  being  kept  at  too 
low  a  temperature  or  from  lack  of  a  sufficient  air-space  round  the 
freezing-point  tube  ;  or  (3)  use  of  a  dirty  tube. 


FREEZING-POINT  METHOD 


349 


depression  of  about  0-3°)  into  the  freezing-point  tube,  and 
find  the  exact  weight  added  by  re-weighing  the  tube  and 
contents. 

Solids  which  cannot  conveniently  be  introduced  in  the 
form  of  powder  should  be  made  into  small  pellets  or  tablets 
by  means  of  a  tablet  press.1  Several  of  these  tablets  are 
placed  in  the  weighing-tube,  weighed,  and  one  or  more  added 
according  to  size.  Liquids  are  weighed  in  a  small  pipette  of 
the  form  shown  in  Fig.  93.  A  piece  of  rubber  tubing  is 
attached  to  the  end  E,  and  a  por- 
tion of  the  liquid  is  transferred  to 
the  freezing-point  tube  by  blowing. 
The  pipette  is  then  weighed  again. 

Stir  vigorously  (holding  the  tube 
at  an  angle  of  about  45°)  until  the 
substance  has  completely  dissolved. 
Then  place  the  tube  directly  in  the 
cooling-bath,  and  stir  the  liquid  at 
intervals  until  it  begins  to  freeze. 
Remove  the  tube  from  the  bath  and, 
after  drying  the  exterior,  warm  with 
the  hand  until  only  a  few  crystals 
remain  unmelted.  Replace  the  tube 
in  position  within  the  wider  tube 
C.  From  this  stage,  proceed  according  to  the  directions 
given  for  determining  the  freezing-point  of  the  pure  solvent. 
The  temperature  rises  at  first  for  perhaps  a  minute,  during 
which  period  the  remaining  crystals  melt ;  it  then  falls 
steadily  until  crystallisation  begins,  when  it  rises  quickly  to 
the  freezing-point  of  the  solution,  and  then  begins  to  fall 
again  slowly. 

The  supercooling,  i.e.,  the  amount  which  the  temperature 
falls  below  the  freezing-point  before  crystallisation  begins, 
must  not  exceed  0-5° ;  if  it  does,  the  observed  freezing-point 
may  be  used  as  a  guide  for  the  next  experiment,  but  must 
not  be  used  for  the  calculation  of  the  molecular  weight.  The 
highest  temperature  reached  after  the  crystallisation  begins 
is  taken  as  the  freezing-point.  If  simple  stirring  is  not 

1  The  "Pigmy"  tablet  -  making  machine,  used  by  pharmacists,  is 
convenient  for  this  purpose. 


FIG.  93. 


350     DETERMINATION  OF  MOLECULAR  WEIGHTS 

sufficient  to  induce  crystallisation  before  the  supercooling 
exceeds  0-5°,  a  nucleus  of  the  solid  solvent  must  be  intro- 
duced through  the  side-tube.  Freeze  some  benzene  in  a 
small  test-tube  and,  by  means  of  a  glass  rod  (cooled  in  the 
same  test-tube),  bring  a  minute  fragment  of  the  solid  solvent 
into  contact  with  the  lower  end  of  the  stirrer  when  the 
supercooling  amounts  to  about  0-3°. 

Determine  the  freezing  -  point  at  least  three  times, 
thawing  the  frozen  material  after  each  measurement,  as 
described  above. 

From  the  depression  of  the  freezing-point  calculate  the 
molecular  weight. 

Add  further  weighed  portions  of  the  substance,  find  the 
freezing-point  after  each  addition,  and  from  each  set  of  data 
calculate  the  molecular  weight. 

Modifications  of  the  above  Procedure  if  a  Beckmann 
Thermometer  is  used. 

The  Beckmann  thermometer  is  described  below,  but  it 
will  be  convenient  to  mention  here  certain  modifications  of 
the  above  procedure  that  are  necessary  if  a  Beckmann 
thermometer  is  used.  The  ordinary  Beckmann  thermometer 
may  be  read  without  difficulty — so  far  as  the  mere  scale 
reading  is  concerned — to  ToVo-°-  This  is  apt  to  lend  a 
spurious  appearance  of  accuracy  to  experiments  with  a 
Beckmann  thermometer  as,  in  reality,  it  is  a  matter  of  diffi- 
culty to  obtain  temperature  measurements  with  it  accurate 
to  Tthj-°.  The  more  important  sources  of  error  are — 

(1)  The  ease  with  which  the  bulb  is  deformed  and  its 

volume  consequently  altered.  This  may  occur 
during  an  experiment  even  with  careful  handling. 

(2)  The  inaccuracy  of  the  scale,  i.e.,  i°  on  the  scale  may 

be  either  more  or  less  than  a  degree.  The  scale  is 
often  inaccurate  and  uneven,  and,  even  if  it  is 
accurate  at  one  temperature,  it  cannot  be  quite 
accurate  at  any  other  temperature  for  which  it  is 
necessary  to  re-set  the  thermometer,  since  this 
involves  alteration  of  the  amount  of  mercury  in 
the  bulb. 


THE  BECKMANN  THERMOMETER  351 

(3)  The  large  heat  capacity  of  the  bulb. 

(4)  The   "sticking"   of    the    mercury   in   the   very    fine 

capillary  of  the  thermometer. 

Apart  from  the  first,  these  errors  can  be  eliminated  either 
by  suitable  modifications  of  the  procedure  or  by  introducing 
the  necessary  corrections.  This,  however,  greatly  complicates 
the  process,  and,  as  highly  accurate  measurements  are  rarely 
required,  it  is  sufficient  for  most  purposes  to  adopt  only  the 
following  modifications  of  the  procedure  already  described. 

(1)  The  amount  of  solvent  taken  should  be  from   15  to 

20  grams.     The  bulb  of  the  thermometer  must  be 
immersed  to  a  depth  of  at  least  I  cm. 

(2)  The  thermometer  must  be  tapped  with  the  finger  (or 

with  a  mechanical  tapper)  prior  to  each  reading. 

The  accuracy  of  the  temperature  measurement,  using  a 
Beckmann  thermometer  under  these  conditions,  is  of  the 
order  of  y^0.  With  a  good  thermometer  graduated  in 
tenths  of  a  degree  the  measurement  of  the  depression  should 
be  accurate  to  between  ^V°  and  w°'  For  ordinary  work  it  is 
doubtful  if  the  increased  time  occupied  in  making  a  freezing- 
point  determination  with  a  Beckmann  thermometer  is  repaid 
by  the  gain  in  accuracy. 

The  Beckmann  Thermometer. 

The  Beckmann  thermometer  (shown  in  Fig.  94)  is 
usually  graduated  in  hundredths  of  a  degree,  and  can  be 
read  with  a  lens  to  y^o-0-  The  scale,  as  a  rule,  covers 
only  about  6°,  but  the  thermometer  may  be  used  at  any 
desired  temperature  by  transferring  mercury  to  or  from  the 
reservoir  at  the  top. 

On  account  of  the  permanent  and  semi  -  permanent 
alterations  produced  in  the  volume  of  the  glass  bulb  if  the 
thermometer  is  subjected  to  any  large  change  in  temperature, 
and  since  any  one  thermometer  cannot  be  accurate  at  two 
widely  different  temperatures,  separate  thermometers  must 
be  reserved  for  freezing-point  and  for  boiling-point 
experiments. 

"Setting"  a  Beckmann  Freezing-point  Thermometer. 
— If  a  Beckmann  thermometer  is  to  be  used  for,  say,  freezing- 


352     DETERMINATION  OF  MOLECULAR  WEIGHTS 

point  determinations  with  benzene  as  solvent,  the  amount  of 
mercury  in  the  bulb  must  be  so  adjusted  that  at  5-5°  the 
top  of  the  thread  will  lie  somewhere  on  the  upper  half  of  the 
scale. 

The  thermometer  bulb  is  placed  in  water,  and  heat  is 
applied  until  the  capillary  is  completely  filled  with  mercury, 
and  a  small  globule  of  mercury  projects  into  the  reservoir. 
The  thermometer  is  then  removed,  inverted,  and  tapped 
gently  until  the  mercury  in  the  reservoir  drops  down  and 
joins  the  thread.  The  thermometer  is  then  cooled  until  the 
temperature  (measured  with  an  ordinary  thermometer)  is 
about  3°  above  the  freezing-point  of  the  solvent,  i.e.,  for 
benzene,  it  is  cooled  in  water  to  about  8-5°.  After  keeping 
it  for  a  few  minutes  at  this  temperature,  the  thermometer 
is  removed  from  the  water,  and  at  once  given  a  sharp  down- 
ward jerk  in  order  to  break  the  thread  of  mercury  at  the 
bend  above  the  reservoir.  If  there  is  difficulty  in  detaching 
the  mercury  in  the  reservoir,  the  thermometer  should  be 
held  vertically  with  the  bulb  2  or  3  inches  above  the  palm  of 
the  left  hand.  The  bulb  is  then  brought  down  fairly  sharply 
on  the  hand,  the  blow  being  delivered  with  the  thermometer 
vertical  throughout.  Any  side  strain  on  the  thermometer 
during  this  operation  may  break  it. 

The  amount  of  mercury  left  in  the  bulb  should  be  such 
that  at  5-5°  the  top  of  the  thread  will  lie  on  the  upper  half 
of  the  scale.  A  beaker  of  water  is  therefore  cooled  to  5-5°, 
using  an  ordinary  thermometer  to  measure  the  temperature, 
and  the  Beckmann  thermometer  is  placed  in  the  water  for 
a  few  minutes.  If  the  top  of  the  thread  lies  on  the  upper 
half  of  the  scale,  the  thermometer  is  "  set "  for  the  experi- 
ment ;  if  it  is  not  on  the  scale  or  is  on  the  lower  portion 
(at  5-5°),  tne  above  "setting"  must  be  more  carefully 
repeated. 

"Setting"  a  Beckmann  Boiling-point  Thermometer. — 
The  procedure  is  identical  with  that  described  above,  except 
that  it  is  necessary  so  to  set  the  thermometer  that  at  the 
boiling-point  of  the  pure  solvent  the  top  of  the  mercury 
thread  comes  on  the  lower  half  of  the  scale.  The  tempera- 
ture of  the  bath  used  in  the  "  setting "  should  therefore  be 
8°  to  10°  above  the  boiling-point  of  the  solvent  to  be  used. 


BECKMANN'S  BOILING-POINT  METHOD  353 

BBCKMANN'S    BOILING-POINT    METHOD. 

The  boiling-point  of  a  solution  is  always  higher  than  that 
of  the  pure  solvent,  and,  with  dilute  solutions,  the  elevation 
of  the  boiling-point  is  proportional  to  the  molecular  con- 
centration of  the  dissolved  substance. 

If  s  grams  of  a  substance  of  molecular  weight  M  are 
dissolved  in  W  grams  of  a  solvent,  and  if  the  elevation  of  the 
boiling-point  is  e°y  the  molecular  weight  of  the  substance 
may  be  found  from  the  formula, 

M  =  K  -4r_ 
e  W 

where  K  is  a  constant  depending  on  the  nature  of  the 
solvent.  K  is  the  amount  by  which  the  boiling-point  would 
be  raised  by  dissolving  i  gram-molecule  of  a  substance  in 
i  gram  of  the  solvent.  It  may  be  found  by  calculation  from 
the  observed  elevation  of  the  boiling-point  produced  by 
dissolving  a  known  weight  of  a  substance  of  known  molecular 
weight  in  a  known  weight  of  the  solvent ;  it  may  also  be 
obtained  from  the  latent  heat  of  evaporation  of  the  solvent. 
The  constants  for  the  commonest  solvents  are  given  in  the 
following  table : — 

Solvent.  Boiling-point.  K. 

Acetone     ....  56-3°  1710 

Benzene     .        .        .  -^"l  80-3°  2650 

Chloroform         .        .         .  61-2°  3660 

Ethyl  acetate     ...  77°  2680 

Ethyl  alcohol     .         .         .  78-3°  1150 

Ethyl  ether         ...  35°  2100 

Methyl  alcohol  ...  67°  860 

Water         ....  100-0°  520 

The  method  cannot  be  used  if  the  dissolved  substance  is 
appreciably  volatile  at  the  boiling-point  of  the  solvent.  In 
general,  the  boiling-point  of  the  solute  must  be  at  least 
120°  above  that  of  the  solvent. 

Apparatus. — The  apparatus  consists  essentially  of  a 
boiling-tube  with  two  side-tubes,  one  of  which,  S,  is  pro- 
vided with  a  glass  stopper.  Inside  the  longer  side-tube  T  is 
fitted  a  small  condenser  C  If  the  solvent  is  hygroscopic,  a 
calcium  chloride  tube  is  attached  to  the  air  inlet  R.  The 

z 


354     DETERMINATION  OF  MOLECULAR  WEIGHTS 


boiling-tube  stands  on  an  asbestos  card,  so  that  the  end  of 
the  tube  closes  a  circular  hole  in  the  asbestos,  but  does  not 
touch  a  sheet  of  wire  gauze  which  is  placed  beneath  the 
asbestos. 

The    boiling -tube    is    protected   from    air -draughts    by 

means  of  a  glass  cylinder  G 
which  is  covered  by  a  sheet 
of  mica  M.  The  sheet  of 
mica  is  perforated  by  a  hole 
which  is  just  large  enough  to 
admit  the  boiling-tube. 

A  thermometer  which  can 
be  read  to  at  least  hundredths 
of  a  degree  is  also  required 
— a  Beckmann  thermometer 
which  can  be  read  to  -g-J-Q-0  or 
Tinnj-0  is  usually  employed. 

Procedure. — Clean  and  dry 
the  boiling-point  tube.  Sus- 
pend it  by  means  of  a  fine  wire 
from  the  hook  of  the  balance, 
and  weigh  the  tube  and  wire. 
Add  10  to  12  c.c.  of  the  sol- 
vent and  weigh  again.  (The 
weighing  of  the  somewhat 
awkward  piece  of  apparatus 
may  be  avoided  by  taking  a 
measured  volume  of  solvent, 
and  using  in  the  calculation 
of  results  the  "volume  con- 
stants" given  on  p.  357.) 

Arrange  the  apparatus  as 
shown  in  Fig.  94,  in  a  place 
where  it  will  not  be  exposed 
to  draughts.  The  bottom  of 
the  thermometer  should  be  at  least  I  cm.  from  the  bottom 
of  the  boiling-point  tube.  If  a  Beckmann  thermometer  is 
used,  it  must  be  "set"  for  the  desired  temperature  as 
described  on  p.  352.  Through  the  side-tube  add  some 
clean,  dry  garnets  (platinum  tetrahedra,  if  available,  are 


FIG.  94. 


BECKMANN'S  BOILING-POINT  METHOD  355 

better)  until  the  thermometer  bulb  is  completely  surrounded 
by  them.  Heat  the  liquid  by  means  of  a  small  flame  until 
it  boils  so  briskly  that  there  is  plentiful  condensation  on  the 
condenser  C,  which  is  kept  cool  by  means  of  a  current  of 
water.  The  point  of  the  condenser  should  be  so  close  to 
the  wall  of  the  side-tube  that  the  condensed  liquid  runs 
away  steadily  without  collecting  into  drops.  (If  drops  form, 
they  cause  fluctuations  in  the  temperature  through  irregular 
cooling  of  the  boiling  liquid.) 

About  twenty  minutes  after  the  liquid  begins  to  boil,  the 
temperature  should  become  constant.  The  variations  of 
temperature  in  the  course  of  five  minutes  should  not  exceed 
ooi.°  If  it  does  not  become  constant  in  about  this  time, 
there  is  some  fault  in  the  arrangement  of  the  apparatus,  or 
the  heating  is  not  properly  adjusted.  (It  is  a  common 
mistake  to  boil  too  gently.)  Examine  the  apparatus  to  be 
sure  that  the  flame  gases  cannot  enter  the  air-mantle,  and 
attend  to  the  other  points  specified  above. 

When  the  boiling-point  of  the  pure  solvent  has  been 
ascertained,  remove  the  flame  until  ebullition  ceases.  Intro- 
duce a  weighed  portion  (cf.  p.  349)  of  the  substance,  preferably 
in  the  form  of  a  tablet,  through  the  side-tube  S,  and  boil 
again  until  the  temperature  becomes  constant.  Note  the 
boiling-point  of  the  solution,  and  from  the  elevation  of 
the  boiling-point  calculate  the  molecular  weight. 

Add  further  portions  of  the  substance,  determine  the 
boiling-point  after  each  addition,  and  from  each  set  of  data 
calculate  the  molecular  weight.  The  amount  added  should 
be  such  that  the  boiling-point  is  raised  about  0-3°  by  each 
addition  of  substance. 

Modification  of  Beckmann's  Method  with  Electrical 
Heating. 

The  boiling-point  tube  used  is  the  same  as  that  already 
described.  It  is  placed  in  a  tall  bottle,  and  the  space 
between  the  bottle  and  tube  is  tightly  packed  with  cotton 
wool  (Fig.  95). 

Through  the  cork  of  the  boiling-tube  pass  two  stout 
nickel  (or  platinum)  wires,  which  are  connected  at  the  lower 


356     DETERMINATION  OF  MOLECULAR  WEIGHTS 

ends  by  a  spiral  of  fine  platinum  wire  (about  01  mm.  in 
diameter).      The   current   necessary  for    boiling    the    liquid 

is  provided  by  a  battery  of 
4  or  5  accumulators.  The 
amount  of  current  required 
varies  for  different  liquids, 
and  must  be  regulated  by 
means  of  a  rheostat.  As 
the  observed  boiling-point 
depends  to  some  extent  on 
the  heat  supplied  (z>.,  on  the 
amount  of  current),  it  is  ad- 
visable to  place  an  ammeter 
in  the  circuit,  and  to  keep 
the  current  constant  through- 
out each  set  of  experiments.  If  the  current  is  so  adjusted 
that  the  liquid  boils  with  sufficient  vigour  to  give  a  steady 
boiling-point  with  the  pure  solvent,  it  is  unnecessary  to  use 
garnets  or  tetrahedra.  The  thermometer  bulb  should  be  at 
least  2  cm.  above  the  heating  spiral. 

LANDSBBRGER'S    BOILING-POINT   METHOD. 

(  Walker-Lumsden  Modification^} 

In  Beckmann's  method,  the  temperature  of  the  flame 
which  heats  the  solution  is  necessarily  higher  than  that  of 
the  boiling  liquid,  and  it  is  difficult  to  avoid  superheating. 
The  use  of  platinum  tetrahedra  or  of  garnets  is  intended  to 
prevent  superheating  and  to  secure  intimate  contact  between 
the  solution  and  the  vapour  of  the  solvent,  which  is  a 
necessary  condition  for  obtaining  the  true  boiling-point,  z>., 
the  temperature  at  which  the  solution  and  the  vapour  of  the 
solvent  are  in  equilibrium. 

By  passing  the  vapour  of  the  boiling  solvent  through  a 
solution,  the  latter  becomes  heated  to  the  boiling-point — but 
not  above  it,  since  the  temperature  of  the  incoming  vapour 
is  originally  at  a  lower  temperature  than  that  of  the  boiling 
solution.  Superheating  is  thus  practically  impossible,  and 
real  equilibrium  between  the  solution  and  the  vapour  of  the 
solvent  is  attained.  This  method  of  heating  is  used  in 


LANDSBERGER'S  BOILING-POINT  METHOD       357 

Landsberger's  apparatus.  For  ordinary  purposes,  where  an 
accuracy  of  about  5  per  cent,  in  the  determination  of  a 
molecular  weight  is  sufficient,  the  following  modification  of 
the  method  is  more  expeditious  and  convenient  than  either 
the  Beckmann  or  the  original  Landsberger  method. 

In  this  method  a  series  of  measurements  at  different 
concentrations  can  be  made  with  a  single  weighed  portion 
of  the  substance.  The  boiling-tube  is  graduated,  and  the 
quantity  of  liquid  is  found  by  noting  its  volume  at 
the  boiling-point.  The  molecular  weight  is  calculated  in 
the  usual  manner,  but  the  molecular  elevation  constant  for 
each  solvent  represents  the  elevation  of  the  boiling-point 
that  would  be  produced  if  a  gram-molecular  weight  of  a 
substance  were  dissolved  in  i  c.c.  (instead  of  i  gram)  of  the 
solvent  at  its  boiling-point.  The  formula  is 

M  =  £—^ 

e  v 

where  e  is  the  elevation  of  the  boiling-point,  s  the  weight  of 
substance,  and  v  the  volume  in  cubic  centimetres  of  the 
solution  at  its  boiling-point.  The  values  of  the  constant  k 
for  the  commonest  solvents  are  given  in  the  following  table  :  — 

Solvent.  Boiling-point.          k. 

Acetone  .         .  ^-  .  56-3°  2220 

Benzene  .              ;  .  .  80-3°  3280 

Chloroform      .  .  .  61-2°  2600 

Ethyl  alcohol  .  .  .  78-3°  1560 

Ethyl  ether      .  .  .  35°  3030 

Methyl  alcohol  .  .  67°  1150 

Water      .  100-0°              540 

The  best  solvents  to  use  are  acetone,  methyl  alcohol,  ethyl 
alcohol,  and  ether.  The  boiling-point  of  the  solute  must 
be  at  least  150°  above  that  of  the  solvent. 

Apparatus. — The  apparatus  required  is  shown  in  Fig.  96. 
The  lower  portion  of  the  boiling-tube  B  is  graduated  (usually 
up  to  30  c.c.).  The  tube  is  expanded  into  a  bulb  above  the 
graduations,  and  is  pierced  by  a  small  hole  at  H.  It  is  fitted 
with  a  cork  which  carries  the  thermometer  (graduated  in 
tenths  of  a  degree)  and  a  tube  A  which  leads  from  a  conical 
flask  F.  The  tube  A  is  sealed  at  the  lower  end,  and  is  then 


358     DETERMINATION  OF  MOLECULAR  WEIGHTS 

perforated  by  a  ring  of  fine  holes  as  near  the  end  as  possible. 
The  boiling-tube  is  completely  surrounded  by  the  glass 
jacket  J,  and  the  jacket  terminates  below  in  a  narrow  tube 
which  is  connected  with  a  condenser  when  a  volatile  solvent, 
such  as  ether,  is  in  use.  With  less  volatile  solvents,  a  flask, 


FIG.  96. 

placed  in  cold  water,  may  be  substituted  for  the  condenser. 
The  flask  F  in  which  the  solvent  is  boiled  is  provided  with  a 
safety  tube  S,  which  should  be  at  least  2  feet  long. 

Procedure. — Clean,  dry,  and  arrange  the  apparatus  as 
shown  in  the  diagram.  The  inlet-tube  should  reach  almost 
to  the  bottom  of  the  boiling-tube,  and  the  fine  holes  at  the 
end  of  it  must  be  at  a  lower  level  than  the  bulb  of  the 


LANDSBERGER'S  BOILING-POINT  METHOD       359 

thermometer.  Introduce  about  10  c.c.  of  the  pure  solvent 
into  the  boiling-tube  B  and  about  150  c.c.  of  it  into  the 
flask  F,  which  is  supported  over  a  wire  gauze.  In  order  to 
ensure  regular  ebullition,  place  a  few  pieces  of  porous  tile 
in  the  flask  F,  and  boil  the  liquid  briskly,  so  that  the  vapour 
passes  steadily  through  the  tube  A  into  the  graduated  tube 
B.  At  first  it  will  all  be  condensed,  but,  when  the  liquid  in 
B  becomes  hot,  some  of  the  vapour  will  pass  through  it  and, 
escaping  through  the  hole  H  into  the  outer  jacket,  will 
condense  and  collect  in  the  flask  below.  The  liquid  in  B 
is  thus  gradually  heated  to  its  boiling-point,  and  the  tempera- 
ture becomes  constant  at  this  point.  When  the  temperature 
is  constant,  read  the  thermometer  as  accurately  as  possible 
with  the  aid  of  a  lens.  This  gives  the  boiling-point  of  the 
pure  solvent. 

The  liquid  in  B  is  now  returned  to  the  flask  F  in  the 
following  manner : — Remove  the  flame  and  at  the  same  time 
close  the  top  of  the  safety  tube  S  by  pressing  a  finger  on  it. 
As  the  flask  cools,  almost  the  whole  of  the  liquid  will  be 
drawn  back  into  it. 

Withdraw  the  cork  from  the  graduated  tube  and  add  a 
weighed  amount  (about  I  gram)  of  the  substance.  Place 
another  piece  of  porous  tile  in  the  flask  and  boil  again. 
When  the  condensed  vapour  is  again  dropping  into  the 
receiver,  and  when  the  volume  of  liquid  in  the  graduated 
tube  has  reached  10  to  12  c.c.,  note  the  temperature 
accurately,  and  then  at  once  extinguish  the  flame  and  dis- 
connect the  flask  F  from  the  graduated  boiling-tube.  Ascertain 
the  volume  of  the  solution  as  follows : — Remove  the  cork 
from  the  graduated  tube,  lift  the  thermometer  and  inlet- 
tube  out  of  the  liquid,  and  read  the  volume  as  accurately  as 
possible.  In  this  way,  the  boiling-point  and  the  correspond- 
ing concentration  of  the  solution  are  obtained. 

Fit  the  apparatus  together  again,  add  another  piece  of 
porous  tile,  and  again  boil  the  liquid  in  the  flask.  The 
temperature  falls  at  first  but  soon  rises  once  more,  reaches 
a  maximum,  and  then  begins  to  fall  again  slowly  and  steadily 
on  account  of  the  progressive  dilution  of  the  solution.  When 
this  stage  is  reached,  again  read  the  boiling-point  and  the 
corresponding  volume  of  the  solution. 


360     DETERMINATION  OF  MOLECULAR  WEIGHTS 

Continue  this  series  of  operations  until  the  volume  has 
reached  25  to  30  c.c.,  and  several  sets  of  boiling-point 
observations  have  been  obtained.  From  each  set  of  data, 
calculate  the  molecular  weight. 

Notes. — A  fresh  piece  of  porous  tile  must  be  placed  in  the 
flask  each  time  the  boiling  is  interrupted,  even  if  only  for  a 
few  seconds. 

Vigorous  boiling  is  necessary.  The  condensed  liquid 
should  collect  in  the  receiver  at  the  rate  of  about  one 
drop  per  second.  (With  ether  as  solvent,  a  condenser  must 
be  inserted  below  the  jacket.) 

Most  of  the  solvents  employed  with  this  method  are 
inflammable;  care  must  be  taken,  therefore,  always  to 
extinguish  the  flame  or  to  remove  it  to  at  least  3  feet  from 
the  apparatus  before  withdrawing  a  cork. 

Quick  working,  especially  in  measuring  the  volume  after 
noting  the  temperature,  is  essential  to  success  with  this 
method. 


APPENDIX 

LIST    OP    COMMON    REAGENTS. 

Unless  specially  mentioned,  it  is  to  be  understood  that  any  reagent 
mentioned  in  the  text  has  the  composition  and  concentration  indicated 
below.  The  concentrations,  etc.,  are  those  adopted  in  the  Chemistry 
Department,  University  of  Edinburgh.  For  convenience,  the  quantity 
necessary  for  the  preparation  of  a  Winchester  of  solution  is  given 
in  each  case.  A  Winchester  contains  about  2400  c.c. 

For  quantitative  work  it  is  usually  necessary  to  prepare  solutions  as 
required,  since  the  bench  solutions,  even  when  prepared  from  the 
purest  chemicals,  usually  contain  appreciable  amounts  of  impurities 
dissolved  from  the  glass. 

Acids  and  Alkalis. 

The  dilute  acids  and  alkalis  are  2  N,  with  the  exception  of  dilute 
sulphuric  acid,  barium  hydroxide,  and  calcium  hydroxide. 

Concentrated  Sulphuric  Acid  (Density  1-84)  is  approximately  36  N. 
It  often  contains  traces  of  iron  and  of  nitric  acid. 

Dilute •  Sulphuric  Acid  (approximately  4  N)  is  prepared  by  diluting 
270  c.c.  of  the  concentrated  acid  to  a  Winchester. 

Concentrated  Nitric  Acid  (Density  1-42). — This  is  the  constant 
boiling-point  acid  and  contains  about  68  per  cent,  of  nitric  acid.  It 
is  about  1 6  N.  The  commonest  impurities  are  chloride  and  sulphate, 

Dilute  Nitric  Acid  (approximately  2  N)  is  prepared  by  diluting  300 
c.c.  of  the  concentrated  acid  to  a  Winchester. 

Concentrated  Hydrochloric  Acid  (Density  1-16)  is  about  10  N. 
It  often  contains  traces  of  iron,  arsenic,  and  sulphate. 

Dilute  Hydrochloric  Acid  (approximately  2  N)  is  prepared  by 
diluting  500  c.c.  of  the  concentrated  acid  to  a  Winchester. 

Acetic  Acid  (approximately  2  N)  is  prepared  by  diluting  280  c.c. 
of  glacial  acetic  acid  (about  17  N)  to  a  Winchester. 

Sodium  Hydroxide  (approximately  2  N)  is  prepared  by  dissolving 
200  grams  of  sodium  hydroxide  ("white  sticks")  in  a  Winchester. 
It  always  contains  carbonate,  and  may  also  contain  chloride,  sulphate, 
alumina,  and  silica. 

861 


362  APPENDIX 

Ammonia  (approximately  2  N)  is  prepared  by  diluting  250  c.c.  of 
concentrated  ammonia  (Density  0-880)  to  a  Winchester.  It  always 
contains  carbonate  and  sometimes  contains  sulphate  and  chloride. 

Ammonium  Carbonate  (approximately  2  N)  is  prepared  by  dis- 
solving 200  grams  of  commercial  ammonium  carbonate  (sesqui- 
carbonate),  together  with  100  c.c.  of  0-880  ammonia,  in  a  Winchester. 

Sodium  Carbonate  (approximately  2  N)  is  prepared  by  dissolving 
250  grams  of  the  anhydrous  salt,  or  680  grams  of  the  decahydrate 
(Na2CO3,  10  H2O)  in  a  Winchester.  It  usually  contains  traces  of  chloride 
and  sulphate,  and  some  samples  are  worthless  on  account  of  the 
amount  of  these  impurities.  It  occasionally  contains  traces  of  ammonia. 

Calcium  Hydroxide  (approximately  0-04  N)  is  a  saturated  solution 
prepared  from  pure  lime. 

Barium  Hydroxide  (approximately  0-4  N)  is  a  saturated  solution  of 
barium  hydroxide  in  water. 

Other  Common  Reagents. 

Alcohol.— Rectified  spirit  which  contains  93  to  95  per  cent,  of 
ethyl  alcohol.  The  "66  over-proof "  spirit  contains  93  per  cent,  of 
alcohol. 

Ammonium  Chloride  (approximately  2  N)  contains  260  grams  of 
ammonium  chloride  in  a  Winchester. 

Ammonium  Oxalate  (approximately  0-5  N)  contains  85  grams  of 
the  crystalline  salt,  (COONH4)2,  H2O,  in  a  Winchester. 

Ammonium  Phosphate  (approximately  0-5  N)  contains  55  grams  of 
di-ammonium  hydrogen  phosphate  in  a  Winchester. 

Ammonium  Sulphide  (approximately  2  N)  is  prepared  by  saturating 
1200  c.c.  of  2  N  ammonia  with  hydrogen  sulphide,  and  then  adding  an 
equal  volume  of  2  N  ammonia. 

Barium  Chloride  (approximately  N)  contains  295  grams  of  the  salt, 
BaCl2,  2H2O,  in  a  Winchester. 

Barium  Nitrate  (approximately  0-5  N)  contains  150  grams  of  the  salt, 
Ba(NO3)2,  in  a  Winchester. 

Bromine  Water  (approximately  0-5  N)  is  a  saturated  solution.  It 
usually  contains  chlorine  and  iodine  as  impurities. 

Calcium  Nitrate  (approximately  N)  contains  200  grams  of  the 
anhydrous  salt  in  a  Winchester. 

Calcium  Sulphate  (approximately  0-03  N)  is  a  saturated  solution. 

Copper  Nitrate  (approximately  0-2  N)  is  prepared  by  dissolving 
70  grams  of  the  crystalline  salt,  Cu(NO3)2,  6H2O,  in  a  Winchester. 

Ferrous  Sulphate  (approximately  N)  is  prepared  by  dissolving  335 
grams  of  the  crystals,  FeSO4,  7H2O,  together  with  300  c.c.  of  dilute 
sulphuric  acid,  in  a  Winchester. 


APPENDIX  363 

Ferric  Chloride  is  prepared  by  dissolving  50  grams  of  commercial 
solid  ferric  chloride  in  100  c.c.  of  concentrated  hydrochloric  acid  and 
diluting  to  a  Winchester. 

Lead  Acetate  (approximately  N)  is  prepared  by  dissolving  455  grams 
of  the  salt,  Pb(C2H3O2)2,  3H2O,  together  with  70  c.c.  of  glacial  acetic 
acid,  in  a  Winchester. 

Mercuric  Chloride  (approximately  02  N)  contains  65  grams  of  the 
salt,  HgCl2,  in  a  Winchester. 

Potassium  Perrocyanide  (approximately  0-2  N)  contains  50  grams 
of  the  salt,  K4Fe(CN)G,  3H2O,  in  a  Winchester. 

Potassium  Chromate  (approximately  02  N)  contains  50  grams  of 
the  salt,  K2CrO4,  in  a  Winchester. 

Potassium  Iodide  (approximately  01  N)  contains  40  grams  of  the 
B.P.  salt  in  a  Winchester. 

Sodium  Acetate  (approximately  N)  contains  325  grams  of  the  salt, 
NaC2H3O2,  3H2O,  in  a  Winchester. 

Stannous  Chloride  (approximately  0-2  N)  is  made  by  dissolving  60 
grams  of  the  salt,  SnCl2,  2H2O,  in  250  c.c.  of  concentrated  hydrochloric 
acid  and  diluting  to  a  Winchester.  A  piece  of  tin  placed  in  each  bottle 
preserves  the  salt  in  the  stannous  state. 

Silver  Nitrate  (approximately  01  N)  contains  40  grams  of  silver 
nitrate  in  a  Winchester. 

SPECIAL  REAGENTS. 

Magnesia  Mixture. — To  70  grams  of  ammonium  chloride  and  60 
grams  of  crystalline  magnesium  chloride,  add  100  c.c.  of  concentrated 
ammonia  and  dilute  to  I  litre.  Filter  a  day  or  two  after  preparation. 

Perchloric  Acid. — Perchloric  acid  is  now  obtainable  commercially  at 
a  reasonable  price.  Methods  of  preparation  are  described  by  Willard 
(/.  Amer.  Ckem.  Soc.,  1912,  34,  1480),  and  Mathers  (Ckem.  Zeit>  1913, 
3V,  363). 

"  Cupferron."— To  60  grams  of  nitrobenzene,  add  I  litre  of  distilled 
water  and  30  grams  of  ammonium  chloride.  Mix  thoroughly  in  a  wide- 
mouthed  bottle  with  an  efficient  stirring  apparatus,  until  a  milky 
emulsion  is  formed.  To  this  emulsion  (constant  stirring)  add  about 
80  grams  of  zinc  dust  (the  amount  depends  on  the  quality),  in  very  small 
portions  at  a  time.  During  the  addition  of  the  zinc  dust  the  temperature 
must  be  kept  between  15°  and  18°  C.  This  may  be  accomplished  by 
adding  pieces  of  ice  to  the  rapidly  whirling  liquid  from  time  to  time. 
Continued  vigorous  stirring  and  the  keeping  of  the  temperature  within 
the  prescribed  limits  are  the  essentials  which  determine  a  good  yield. 
Continue  the  reduction  until  the  odour  of  nitrobenzene  vanishes.  The 
time  required  for  the  reduction  depends  on  the  value  of  the  zinc  dust. 
It  usually  takes  half  an  hour  to  reduce  60  grams  of  nitrobenzene. 
Filter  off  the  white  zinc  hydroxide,  using  the  filter-pump  ;  cool  the  filtrate 
to  o°  with  ice,  and  add  sufficient  common  salt  to  saturate  the  solution. 


364  APPENDIX 

After  a  short  time,  a  thick  mass  of  snow-white  crystals  separates. 
Filter  immediately,  using  a  Biichner  funnel,  and  dry  the  crystals  between 
filter  paper.  The  yield  of  phenylhydroxylamine  is  usually  about  70  to 
85  per  cent,  of  the  theory. 

A s  phenylhydroxylamine  solutions  are  vigorous  skin  poisons,  and  may 
pass  through  the  unbroken  skin  into  the  blood,  the  hands  should  be  washed 
with  water  and  alcohol,  in  case  they  come  in  contact  with  such  solutions. 

Dry  the  freshly  prepared  phenylhydroxylamine  for  an  hour  between 
filter  paper,  and  then  dissolve  in  300  to  500  c.c.  of  commercial  ether. 
Filter  the  ether  solution  through  a  dry  filter,  and  cool  to  o°.  Into  this 
cold  solution  pass  dry  ammonia  gas  for  about  ten  minutes,  and  then 
add  somewhat  more  than  the  theoretical  amount  (more  than  0-5  gram- 
molecule)  of  fresh  amyl  nitrite  all  at  once.  The  clear  solution  will 
suddenly  get  hot,  and  the  entire  vessel  will  be  filled  with  snow-white 
crystals  of  the  ammonium  salt  of  nitroso-phenylhydroxylamine. 

Preserve  in  a  stoppered  bottle  with  addition  of  a  lump  of  ammonium 
carbonate. 

Hydrochloroplatinic  Acid. — This  is  usually  obtained  as  the  hydrate 
H2PtCl6,  6H2O,  which  contains  37-66  per  cent,  of  platinum.  In  order  to 
prepare  a  solution  containing  10  per  cent,  of  platinum  (10  grams  of 
platinum  per  100  c.c.  of  solution),  dissolve  I  oz.  of  this  hydrate  in 
about  50  c.c.  of  water,  filter,  and  wash  the  vessel  and  filter  two  or  three 
times  with  water.  Dilute  the  filtrate  and  washings  to  106  c.c. 

The  preparation  of  this  "  10  per  cent. "  solution  from  commercial 
platinum  is  described  by  Treadwell  (Qualitative  Analysis,  p.  236). 

Indicator   Solutions. 

Litmus  is  prepared  by  dissolving  I  gram  of  azolitmin  in  I  litre  of 
water. 

Methyl  Orange  is  prepared  by  dissolving  0-05  gram  of  the  solid  in 
i  litre  of  water. 

Methyl  Red  is  prepared  by  dissolving  0-05  gram  of  the  solid  in 
500  c.c.  of  alcohol,  and  diluting  to  I  litre  with  water. 

Phenolphthalein  is  prepared  by  dissolving  I  gram  of  the  solid  in 
500  c.c.  of  alcohol,  and  diluting  to  I  litre  with  water. 

STANDARD  SOLUTIONS  FOR  ANALYSIS. 

It  is  customary  for  beginners  to  perform  their  first  quantitative 
exercises  with  pure  salts  of  known  composition.  There  are  many  objec- 
tions to  this  system — the  most  serious  is  that  the  exercise  does  not 
imitate  the  conditions  met  with  in  ordinary  practice.  For  example, 
when  working  with  a  known  quantity  of  material,  the  problem  of  how 
much  precipitant  to  add  does  not,  as  a  rule,  present  any  difficulty  ;  in 
this  important  particular,  therefore,  the  exercise  lacks  much  of  the 
educational  value  it  should  possess.  For  this  reason  alone,  it  is  desirable 
that  all  quantitative  exercises  (even  the  first)  should  be  performed  with 
solids  or  solutions  of  unknown  composition. 


APPENDIX 


365 


The  most  convenient  system  whereby  a  number  of  students  can  be 
provided  with  different  exercises  is  to  use  standard  solutions,  and  the 
following  list  may  be  found  convenient.  All  the  solutions  mentioned 
below  can  be  prepared  by  weight  from  substances  which  are  obtainable 
commercially  in  a  pure  state.  The  quantities  given  are  the  amounts 
required  for  the  preparation  of  I  litre  of  solution.  With  these  concentra- 
tions, 20  to  30  c.c.  is  a  suitable  quantity  for  an  analysis.  For  small 
classes,  the  portions  for  analysis  may  be  measured  with  a  pipette  or 
burette  j  for  larger  classes,  it  is  an  advantage  to  store  the  solution  in 
a  bottle  with  a  burette  permanently  attached,  as  shown  in  Fig.  23,  on 
p.  62. 

Aluminium        .     70  grams  of  ammonia  alum,  A1(NH4)(SO4)2,  I2H2O. 

Ammonia  .  40  grams  of  ammonium  chloride,  NH4C1.  Dry  in 
a  desiccator  before  weighing. 

Arsenic  .  .  6  grams  of  arsenious  oxide,  As2O3,  dissolved  in  dilute 
hydrochloric  acid. 

Calcium  .  .  20  grams  of  pure  calcspar,  CaCO3,  dissolved  in  dilute 
hydrochloric  acid. 

Carbonate  .  100  grams  of  uneffloresced  crystals  of  sodium  car- 
bonate, Na2CO3,  ioH2O. 

Chloride  .         .15  grams  of  barium  chloride,  BaCl2,  2H2O. 

Chromate          .     20  grams  of  potassium  dichromate,  K2Cr2O7. 

Copper     .         .     30  grams  of  copper  sulphate,  CuSO4,  5 H2O. 

Iodide       .         .     25  grams  of  potassium  iodide,  KI.     (Dry.) 

Iron          .         .     50  grams  of  ferric  alum,  Fe  (NH4)(SO4)2,  I2H2O. 

Lead  .  .  20  grams  of  lead  nitrate,  Pb(NO3)2.  It  is  advisable 
to  recrystallise  this  salt  from  a  dilute  nitric  acid 
solution,  and  dry. 

Magnesium      .     25  grams  of  magnesium  sulphate,  MgSO4,  7H2O. 

Manganese  .  Dissolve  20  grams  of  pure  potassium  permanganate 
in  water,  and  pass  sulphur  dioxide  until  it  is 
completely  decolorised  and  the  precipitated 
manganese  dioxide  has  dissolved.  Boil  until 
free  from  sulphur  dioxide,  and  dilute  to  I  litre. 

Mercury  .  .10  grams  of  red  mercuric  oxide,  HgO,  dissolved  in 
concentrated  nitric  acid,  boiled,  and  diluted  to 
i  litre. 

Nickel      .        .     35  grams  of  nickel  sulphate,  NiSO4.,  7H2O. 

Nitrate  .  .  For  reduction  method  : — 50  grams  of  potassium  nitrate. 
For  oxidation  method  : — 4  grams  of  potassium 
nitrate. 

Phosphate         .     15  grams  of  sodium  phosphate,  Na2HPO4, 12H2O. 

Potassium         .     1 5  grams  of  potassium  chloride.     (Dry.) 

Silver        .        .     20  grams  of  silver  nitrate,  AgNO3. 

Sulphate  .        .     25  grams  of  magnesium  sulphate,  MgSO4,7H2O. 

Zinc  ,         .     30  grams  of  zinc  sulphate,  ZnSO4,  7H2O. 


366 


APPENDIX 


TYPICAL    ANALYSES. 

Various  Glasses. 


Si02. 

PbO. 

Al./>3. 

Fe.203. 

CaO. 

Na2O. 

K2O. 

Total. 

Table  glass 

70-61 

0-70 

4-92 

11-95 

12-28 

100-46 

'Table  glass 

70-04 

1-81 

1-27 

13-11 

13-98 

100-21 

Plate  glass 

71-72 

1-29 

0-13 

15-54 

11-49 

100-17 

Mirror  glass 

77-35 

... 

... 

... 

1-25 

15-05 

6-33 

99-98 

White  bottle 

glass*     . 

68-64 

2-83 

0-71 

14-94 

13-01 

100-37 

Flint  glass 

42-65 

43-17 

0-28 

0-28 

13-89 

100-27 

*  Contained  also  0-24  per  cent,  of  MnO. 
colourless  glasses.) 


(Traces  of  Mn  are  almost  invariably  present  in 


Various  Silicates. 


Si02. 

MgO. 

A1203. 

Fe203. 

CaO. 

K20. 

Na2O. 

H20. 

Total. 

Albite       . 

68-80 

19-43 

0-20 

nil 

11-68 

100-11 

Albite      . 

67-99 

19-23 

1-84 

1-25 

9-69 

100-00 

Albite       . 

68-40 

19-89 

0-90 

10-69 

99-88 

Orthoclase 

66-56 

19-18 

0-52 

6-94 

6-56 

99-76 

Orthoclase 

66-58 

21-26 

1-18 

0-76 

10-26 

0-16 

100-20 

A  clay 

63-69 

17-02 

10-18 

0-97 

4-02 

4-05 

99-93 

Talc 

63-42 

31-49 

0-57 

4-38 

99-86 

Talc 

62-78 

31-16 

1-85 

4-32 

100-11 

Iron  Pyrites. 


s. 

Cu. 

Pe. 

Mn. 

Zn. 

Insoluble 
residue. 

Total. 

43-03 

2-50 

?9'54 

0-06 

0-42 

14-68 

100-23 

42-59 

1-49 

40-11 

0-03 

072 

15-01 

99-95 

53-37 

2-39 

44-47 

100-23 

52-71 

0-24 

44-23 

2-58 

99-76 

48-73 

... 

42-94 

0-18 

7-82 

99-67 

Dolomite. 


CaO. 

MgO. 

C02. 

Fe20:, 
+A1203. 

FeO. 

H20. 

Insoluble 
residue. 

Total. 

29-51 

20-29 

47-22 

0-82 

1-05 

1-33 

100-22 

30-75 

25-18 

42-01 

6-83 

0-07 

1-30 

100-14 

29-61 

12-94 

44-72 

... 

12-99 

... 

... 

100-26 

APPENDIX 


367 


Cassiterite. 


SnO2. 

Fe203. 

CaO. 

SiO2. 

Total. 

9874 

0-12 

0-41 

0'19 

99-46 

Cassiterite  often  contains  traces  of  As  and  Zn. 


Garnet  (Pyrope). 


Si02. 

A1203. 

Fe2O3. 

FeO. 

MnO. 

MgO. 

CaO. 

H20. 

Total.    | 

40-92 

22-45 

5-46 

8-}l 

0-46 

17-85 

5'04 

o-io 

100-39 

Traces  of  Cr  are  usually  found  in  garnets. 


Manganese  Minerals. 


MnO2. 

MnO. 

FesAj- 

BaO. 

H20. 

Insoluble 
residue. 

Total. 

Pyrolusite  . 
Pyrolusite  . 

86-45 
69-06 

6-02 
18-16 

0-93 

1-31 

1-22 

4-11 
0-55 

100-04 

Manganite         « 

48-47 

42-03 

... 

... 

7-41 

1-72 

99-63 

Zinc  Blende. 


s. 

Zn. 

Cd. 

Fe. 

Mn.    ' 

Pb. 

Total. 

32-98 
33-25 

64-92 
50-02 

1-05 
0'30 

0-57 
15-44 

0-37 
nil 

0-15 
1-01 

100-04 
100-02 

Copper  Pyrites. 


s. 

Fe. 

Cu. 

Ag. 

Pb. 

Insoluble 
residue. 

Total. 

30-50 

21-08 

48-40 

trace 

99-98 

33-18 

32-65 

32-79 

nil 

0-35 

1-04 

100-01 

36-15 

29-34 

32-25 

nil 

0-30 

2-09 

100-13 

368 


APPENDIX 


Density  and  Concentration  of  Hydrochloric  Acid  at  15C 

(Normal  HC1  =  36-47  grams  per  litre.} 


100  grams 

1  litre 

100  grams. 

1  litre 

Density 

contain 

contains 

Nor- 

Density 

contain 

contains 

Nor- 

15°/4°. 

grams 

grams 

mality. 

1574°. 

grams 

grams 

mality. 

HC1. 

HC1. 

HC1. 

HC1. 

1-010 

2-14 

22 

0-6 

1-110 

21-9 

243 

6-7 

1-020 

4-13 

42 

1-2 

1-120 

23-8 

267 

7-3 

1-080 

6-15 

64 

1-8 

1-180 

25-7 

291 

8-0 

1-040 

8-16 

85 

2-3 

1-140 

27-7 

315 

8-6 

1-050 

10-17 

107 

2-9 

1-160 

29-6 

340 

9-3 

1-060 

12-19 

129 

3-5 

1-160 

31-5 

366 

10-0 

1-070 

14-17 

152 

4-2 

1-170 

33-5 

392 

10-8 

1-080 

16-15 

174 

4-8 

1-180 

35-4 

418 

11-7 

1-090 

18-1 

197 

5*4 

1-180 

37-2 

443 

12-1 

1-100 

20-0 

220 

6-0 

1-200 

39-1 

469 

12-9 

Density  and  Concentration  of  Sulphuric  Acid  at  15°. 

(Normal  H2SO4  =  49-04  grams  per  litre.} 


Density 

1574°. 

100  grams 
contain 
grams 
H2S04. 

1  litre 
contains 
grams 
H2S04. 

Nor- 
mality. 

Density 

1574°. 

100  grams 
contain 
grams 
H2S04. 

1  litre 
contains 
grams 
H2S04. 

Nor- 
mality. 

1-006 

1 

10-1 

0-20 

1-449 

55 

797 

16-25 

1-018 

2 

20-3 

0-41 

1-502 

60 

901 

18-38 

1-020 

3 

30-4 

0-62 

1-558 

65 

1013 

20-65 

1-026 

4 

41-0 

0-84 

1-615 

70 

1130 

23-05 

1-088 

5 

51-6 

1-05 

1-674 

75 

1248 

25-60 

1-040 

6 

62-4 

1-27 

1-782 

80 

1386 

28-26 

1-047 

7 

73-3 

1-49 

1-784 

85 

1520 

30-92 

1-054 

8 

84-3 

1-72 

1-820 

90 

1540 

33-40 

1-061 

9 

95-5 

1-95 

1-825 

91 

1660 

33-86 

1-068 

10 

106-8 

2-18 

1-829 

92 

1680 

34-32 

1-104 

15 

160-6 

3-38 

1-888 

93 

1710 

34-76 

1-142 

20 

228 

4-66 

1-886 

94 

1730 

35-20 

1-182 

25 

296 

6-02 

1-889 

95 

1750 

35-62 

1-222 

30 

367 

7-48 

1-841 

96 

1770 

36-03 

1-264 

35 

444 

9-02 

1-841 

97 

1790 

36-42 

1-806 

40 

522 

10-66 

1-841 

98 

1800 

36-79 

1-851 

45 

608 

12-40 

1-889 

99 

1820 

37-13 

1-399 

50 

700 

14-26 

(1-886) 

100 

(1836) 

(37-4) 

APPENDIX 


369 


Density  and  Concentration  of  Nitric  Acid  at  15°. 
(Normal  HNO3  =  63-02  grams  per  litre.) 


Density 

15°/4°. 

100  grams 
contain 
grams 
HN03. 

1  litre 
contains 
grams 
HNO3. 

Nor- 
mality. 

Density 

1574°. 

100  grams 
contain 
grams 
HN03. 

1  litre 
contains 
grams 
HN03. 

Nor- 
mality. 

1-010 

1-90 

19 

0-80 

1-280 

44-41 

568 

9-01 

1-020 

3-70 

38 

0-60 

1-300 

47-49 

617 

9-8 

1-040 

7-26 

75 

1-19 

1-320 

50-71 

669 

10-6 

1-060 

10-68 

113 

1-79 

1-340 

54-07 

725 

11-5 

1-080 

13-95 

151 

2-35 

1-360 

57-57          783 

12-4 

1-100 

17-11 

188 

2-99 

1-380 

61-27          846 

13-4 

1-120 

20-23 

227 

3-60 

1-400 

65-30          914 

14-5 

1-140 

23-31 

266 

4-22 

1-420 

69-80          991 

15-7 

1-160 

26-36 

306 

4-84 

1-440   j     74-68        1075         17'1 

1-180 

29-38 

347 

5-51 

1-460 

79-98        1168         18-5 

1-200 

32-36 

388 

6-16 

1-480 

86-05        1274         20-2 

1-220 

35-28 

430 

6-83 

1-500 

94-1          1410 

22-4 

1-240 

38-29 

475 

7-54 

1-510 

98-1          1480 

23-5 

1-260 

41-34 

521 

8-25 

1-520 

99-7 

1515 

24-0 

Density  and  Concentration  of  Perchloric  Acid  at  15°. 


Density 

15°/4% 

100  grams 
contain 
grams 
HC104. 

1  litre 
contains 
grams 
HC1O4. 

Density 

1574°. 

100  grams 
contain 
grams 
HC104. 

1  litre 
contains 
grams 
HC1O4. 

1-030 
1-060 
1-090 
1-120 
1-150 
1-180 
1-210 
• 

5-25 
10-06 
14-56 
18-88 
22-99 
26-82 
30-45 

54 
107 
159 
212 
264 
316 
369 

1-240 
1-270 
1-800 
1-860 
1-420 
1-540 
1-675 

33-85 
37-08 
40-10 
45-71 
50-91 
60-04 
70-15 

420 
471 
521 
622 
723 
925 
1175 

2  A 


370 


APPENDIX 


Density  and  Concentration 
of  Potassium  Hydroxide 
at  15°. 


Density  and  Concentration 
of  Sodium  Hydroxide  at 
15°. 


100  grams 

1  litre 

Density 

contain 

contains 

15°/4°. 

grams 

grams 

KOH. 

KOH. 

1-033 

5 

52 

1-082 

10 

108 

1-184 

15 

178 

1-176 

20 

235 

1-230 

25 

307 

1-287 

30 

386 

1-346 

35 

471 

1-411 

40 

564 

1-473 

45 

663 

1-538 

50 

769 

100  grams 

1  litre 

Density 

contain 

contains 

1574°. 

grams 
NaOH. 

grams 
NaOH. 

1-058 

F 

53 

1-113 

10 

111 

1-170 

15 

175 

1-224 

20 

245 

'279 

25 

320 

•332 

30 

400 

•383 

35 

484 

•433 

40 

573 

•481 

45 

666 

1-529 

50 

765 

Density  and  Concentration  of  Ammonia  Solutions  at  15C 

(Normal  NH3  =  17-03  grams  per  litre.} 


Density 

100  grams 
contain 

1  litre 
contains 

Nor- 

Density 

100  grams 
contain 

1  litre 
contains 

Nor- 

1574°. 

grams 
NH3. 

grams 
NH3. 

mality. 

1574° 

grams 
NH3. 

grams 
NH3. 

mality. 

0-990 

2-31 

22'9 

1-3 

0-930 

18-64 

173-4 

10-2 

0-980 

4-80 

47-0 

2-8 

0-920 

21-75 

210-1 

11-8 

0-970 

7-31 

70-9 

4-2 

0'910 

24-99 

227-4 

13-4 

0-960 

9-91 

95-1 

5-6 

0-900 

28-33 

255-0 

15-0 

0-950 

12-74 

121-0 

7-1 

0-890 

31-75 

282-6 

16-6 

0-940 

15-63 

146-9 

8-6 

0-880 

35-70 

314-2 

18-5 

Density  of  Aqueous  Alcohol  at  15°. 


Density 
1574°. 

Grams  of 
alcohol 
per  100 
grams. 

Grams  of 
alcohol 
per  litre. 

Density 
1574°. 

Grams  of 
alcohol 
per  100 
grams. 

Grams  of 
alcohol 
per  litre. 

0-983 

10 

98 

0-872 

70 

610 

0-971 

20 

194 

0-860 

75 

645 

0-957 

30 

287 

0-848 

80 

678 

0-939 

40 

376 

0-835 

85 

710 

0-918 

50 

459 

0-822 

90 

740 

0-895 

60 

537 

0-808 

95 

768 

0-840 

65 

546 

0-793 

100 

793 

APPENDIX 


371 


Weight  of  1  Litre  of  Various  Dry  Gases  at  0°  and  760  mm. 


Grams. 

Grams. 

Air      .         .        .        . 
Carbon  monoxide 
Carbon  dioxide  . 
Hydrogen  .        .        , 

1-2928 
1-2506 
1-9652 
0-0900 

Methane   . 
Nitric  oxide      . 
Nitrogen  .        .        . 
Oxygen     . 

0-7160 
1-3412 
1-2505 
1-4292 

Vapour  Pressure  of  Water. 


Tempera- 
ture. 

Vapour 
Pressure. 

Tempera- 
ture. 

Vapour 
Pressure. 

Tempera- 
ture. 

Vapour 
Pressure. 

rnm. 

mm. 

mm. 

4° 

6-1 

14° 

11-9 

24° 

22-2 

5° 

6-5 

15° 

12-7 

25° 

23-5 

6° 

7-0 

16° 

13-6 

26° 

25-0 

7° 

7-5 

17° 

14-4 

27° 

26-5 

8° 

8-0 

18° 

15-4 

28° 

28-1 

9° 

8-6 

19° 

16-4 

29° 

29-8 

10° 

9-2 

20° 

17'4 

30° 

31-5 

11° 

9-8 

21° 

18-5 

31° 

33-4 

12° 

10-5 

22° 

19-7 

32° 

35-4 

13° 

11-2 

23° 

20-9 

33° 

37-4 

Vapour  Pressure  of  Potassium  Hydroxide  Solutions 


Temperature. 

Vapour  pressure  of 
40  per  cent,  solution. 

Vapour  pressure  of 
50  per  cent,  solution. 

mm. 

mm. 

10° 

6'5 

5'6 

12° 

7-5 

6-5 

14° 

8-4 

7'3 

16° 

9-6 

8-3 

18° 

10-9 

9-5 

20° 

12-4 

10-8 

22° 

13-9 

12-1 

The  "  40  per  cent."  solution  is  one  containirg  40  grams  of  potassium  hydroxide 
per  100  grams  of  water,  and  the  "50  per  cent."  solution  one  containing  50  grams 
of  potassium  hydroxide  per  100  grams  of  water. 


Logarithms. 


100 
101 
102 
103 
104 

0 

0000 
0043 
0086 
0128 
0170 

1 

0004 
0048 
0090 
0133 
0175 

2 

3 

0013 
0056 
0099 
0141 
0183 

4 

0017 
0060 
0103 
0145 
0187 

5 

0022 
0065 
0107 
0149 
0191 

6 

7 

0030 
0073 
0116 
0158 
0199 

8 

0035 
0077 
0120 
0162 
0204 

9 

0009 
0052 
0095 
0137 
0179 

0026 
0069 
0111 
0154 
0195 

0039 
0082 
0124 
0166 
0208 

105 
106 
107 
108 
109 
110 

11 
12 
13 
14 

0212 
0253 
0294 
0334 
0374 
0414 

0414 
0792 
1139 
1461 

0216 
0257 
0298 
0338 
0378 
0418 

0453 
0828 
1173 
1492 

0220 
0261 
0302 
0342 
0382 
0422 

0492 
0864 
1206 
1523 

0224 
0265 
0306 
0346 
0386 
0426 

0531 
0899 
1239 
1553 

0228 
0269 
0310 
0350 
0390 
0430 

0569 
0934 
1271 
1584 

0233 
0273 
0314 
0354 
0394 
0434 

0607 
0969 
1303 
1614 

0237 
0278 
0318 
0358 
0398 
0438 

0645 
1004 
1335 
1644 

0241 
0282 
0322 
0362 
0402 
0441 

0682 
1038 
1367 
1673 

0245 
0286 
0326 
0366 
0406 
0445 

0719 
1072 
1399 
1703 

0249 
0290 
0330 
0370 
0410 
0449 

0755 
1106 
1430 
1732 

1  2  3 

456 

789 

4  8  11 
3  7  10 
3  6  10 
369 

15  19  23 
14  17  21 
13  16  19 

12  15  18 

26  30  34 
24  28  31 
23  26  29 

21  24  27 

15 
16 
17 
18 
19 

1761 
2041 
2304 
2553 

2788 

1790 
2068 
2330 
2577 
2810 

1818 
2095 
2355 
2601 
2833 

1847 
2122 
2380 
2625 
2856 

1875 
2148 
2405 
2648 
2878 

1903 
2175 
2430 
2672 
2900 

1931 
2201 
2455 
2695 
2923 

1959 
2227 

2480 
2718 
2945 

1987 
2253 
2504 
2742 
2967 

2014 
2279 
2529 
2765 
2989 

368 
358 
2  5  7 
2  5  7 
247 

11  14  17 
11  13  16 
10  12  15 
9  12  14 
9  11  13 

20  22  25 
18  21  24 
17  '20  22 
16  19  21 
16  18  20 

20 
21 
22 
23 
24 

3010 
3222 
3424 
3617 
3802 

3032 
3243 
3444 
3636 
3820 

3054 
3263 
3464 
3655 

3838 

3075 
3284 
3483 
3674 
3856 

3096 
3304 
3502 
3692 
3874 

3118 
3324 
3522 
3711 
3892 

3139 
3345 
3541 
3729 
3909 

3160 
3365 
3560 
3747 
3927 

3181 
3385 
3579 
3766 
3945 

3201 
3404 
3598 
3784 
3962 

246 
246 
246 
246 
245 

8  11  13 
8  10  12 
8  10  12 
7   9  11 
7   9  11 

15  17  19 
14  16  18 
14  15  17 
13  15  17 
12  14  16 

25 
26 
27 
28 
29 

3979 
4150 
4314 
4472 
4624 

3997 
4166 
4330 
4487 
4639 

4014 
4183 
4346 
4502 
4654 

4031 
4200 
4362 
4518 
4669 

4048 
4216 
4378 
4533 
4683 

4065 
4232 
4393 
4548 
4698 

4082 
4249 
4409 
4564 
4713 

4099 
4265 
4425 
4579 
4728 

4116 
4281 
4440 
4594 
4742 

4133 
4298 
4456 
4609 
4757 

235 
235 
235 
235 
1  3  4 

7   9  10 
7   8  10 
689 
689 
679 

12  14  15 
11  13  15 
11  13  14 
11  12  14 
10  12  13 

30 
31 
32 
33 
34 

4771 
4914 
5051 
5185 
5315 

4786 
4928 
5065 
5198 
5328 

4800 
4942 
5079 
5211 
^340 

4814 
4955 
5092 
5224 
5353 

4829 
4969 
5105 
5237 
5366 

4843 
4983 
5119 
5250 
5378 

4857 
4997 
5132 
5263 
5391 

4871 
5011 
5145 
5276 
5403 

4886 
5024 
5159 
5289 
5416 

4900 
5038 
5172 
5302 
5428 

134 
1  3  4 
1  3  4 
1  3  4 
1  3  4 

679 

678 
578 
568 
568 

10  11  13 
10  11  12 
9  11  12 
9  10  12 
9  10  11 

35 
36 
37 
38 
39 

5441 
5563 
5682 
5798 
5911 

5453 
5575 
5694 
5809 
5922 

5465 
5587 
5705 
5821 
5933 

5478 
5599 
5717 
5832 
5944 

5490 
5611 
5729 
5843 
5955 

5502 
5623 
5740 
5855 
5966 

5514 
5635 
5752 
5866 
5977 

5527 
5647 
5763 

5877 
5988 

5539 
5658 
5775 
5888 
5999 

5551 
5670 
5786 
5899 
6010 

1  2  4 
1  2  4 
123 
1  2  3 
123 

567 
567 
567 
56.7 
4   5   7 

9  10  11 
8  10  11 
8   9  10 
8   9  10 
8   9  10 

40 
41 
42 
43 
44 

6021 
6128 
6232 
6335 
6435 

6031 
6138 
6243 
6345 
6444 

6042 
6149 
6253 
6355 
6454 

6053 
6160 
6263 
6365 
6464 

6064 
6170 
6274 
6375 
6474 

6075 
6180 
6284 
6385 
6484 

6085 
6191 
6294 
6395 
6493 

6096 
6201 
6304 
6405 
6503 

6107 
6212 
6314 
6415 
6513 

6117 
6222 
6325 
6425 
6522 

123 
123 
1  2  3 
1  2  3 
123 

456 
456 
456 
456 
456 

8   9  10 
789 
789 
789 
789 

45 
46 
47 
48 
49 

6532 
6628 
6721 
6812 
6902 

6542 
6637 
6730 
6821 
6911 

6551 
6646 
6739 
6830 
6920 

6561 
6656 
6749 
6839 
6928 

6571 
6665 
6758 
6848 
6937 

6580 
6675 
6767 
6857 
6946 

6590 
6684 
6776 
6866 
6955 

6599 
6693 
6785 
6875 
6964 

66096618 
67026712 
67946803 
68846893 
69726981 

123 
123 
123 
1  2  3 
123 

456 
456 
455 
445 
445 

789 
7   7   8 
678 
678 
678 

872 


Logarithms. 


0 

1 

2 

3 

4 

5 

6 

7 

S 

9 

1  2  3 

456 

789 

50 

6990 

6998 

7007 

7016 

7024  7033 

7042 

7050 

7059 

7067 

1     2     3 

345 

678 

51 

7076 

7084 

7093 

7101 

71107118 

7126 

7135 

7143 

7152 

1     2     3 

345 

678 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

122 

345 

677 

53 

7243 

7251 

7259 

7267 

7275  7284 

7292 

7300 

7308 

7316 

1     2     2 

345 

667 

54 

7324 

7332 

7340 

7348 

7356 

7364 

7372 

7380 

7388 

7396 

1     2     2 

345 

667 

55 

74047412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

1     2     2 

345 

567 

56 

7482  7490 

7497 

7505 

751317520 

7528 

7536 

7543 

7551 

1     2     2 

345 

5     6     7 

57 

75597566 

7574 

7582 

7589  7597 

7604 

7612 

7619 

7627 

1     2     2 

345 

567 

58 

7634J7642 

7649 

7657 

76647672 

7679 

7686 

7694 

7701 

1     1     2 

344 

567 

59 

77097716 

7723 

7731 

77387745 

7752 

7760 

7767 

7774 

112 

344 

567 

60 

7782 

7789 

7796 

7803 

78107818 

7825 

7832 

7839 

7846 

1     1     2 

344 

566 

61 

7853 

7860 

7868 

7875 

7882 

7889 

7896 

7903 

7910 

7917 

112 

344 

566 

62 

7924 

7931 

7938 

7945 

79527959 

7966 

7973 

79807987 

1     1     2 

334 

566 

63 

7993 

8000 

8007 

8014 

8021  8028 

8035 

8041 

8048  8055 

1     1     2 

334 

556 

64 

8062 

8069 

8075 

8082 

80898096 

8102 

8109 

81168122 

112 

334 

556 

65 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8176 

81828189 

1     1     2 

334 

556 

66 

8195 

8202 

8209 

8215 

8222  8228 

8235 

8241182488254 

1     1     2 

334 

556 

67 

8261 

8267 

8274 

8280 

82873293 

8299 

8306[83128319 

1     1     2 

334 

556 

68 

8325 

8331 

8338 

8344 

8351  8357 

8363 

837083768382 

1     1     2 

334 

456 

69 

8388 

8395 

8401 

8407 

84148420 

8426 

843284398445 

112 

234 

456 

70 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

849485008506 

1     1     2 

234 

456 

71 

8513 

8519,8525 

8531 

85378543 

8549 

8555  8561  8567 

112 

234 

455 

72 

8573 

8579  8585 

8591 

8597  8603 

8609 

861586218627 

112 

234 

455 

73 
74 

8633 
8692 

8639 

8698 

8645 
8704 

8651 
8710 

36578663 
87168722 

8669 
8727 

8675  8681  8686 

873387398745 

112 
112 

234 
234 

455 
455 

75 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

87978802 

1     1     2 

233 

455 

76    8808 

8814 

8820 

8825 

8831 

8837 

8842  8848  8854!8859 

112 

233 

455 

77   8865 

8871 

8876 

8882 

8887 

8893889989048910^915 

1     1     2 

233 

445 

78   8921 

8927 

8932 

8938 

8943 

89498954896089658971 

1     1     2 

233 

445 

79 

8976 

8982 

8987 

8993 

8998 

9004  9009 

9015 

9020  9025 

1     1     2 

233 

4     4     f. 

80    90319036 

9042 

9047 

9053 

9058  9063  9069  9074'9079 

1     1     2 

233 

445 

81    90859090 

9096 

9101 

9106  9112  9117:9122  9128  9133 

1     1     2 

233 

445 

82    91389143 

9149 

9154 

915991659170:9175 

91809186 

1     1     2 

233 

445 

83   91919196 

9201 

9206 

9212921792229227 

9232  9238 

1     1     2 

233 

445 

84  ,92439248 

9253 

9258 

92639269,92749279 

92849289 

1     1     2 

233 

445 

85 

9294  9299 

9304 

9309 

9315 

9320,9325 

9330 

93359340 

1     1     2 

233 

445 

86 

9345  9350 

9355 

9360 

9365 

93709375 

9380 

93859390 

1     1     2 

233 

445 

87 

9395  9400 

9405 

9410 

9415 

9420  9425 

9430 

9435'9440 

Oil 

223 

344 

88 

9445 

9450 

9455 

9460 

9465 

9469 

9474 

9479 

9484|9489 

0     1     1 

223 

344 

89 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

95339538 

Oil 

223 

344 

90 

9542,9547 

9552 

9557 

9562 

9566 

9571 

9576 

9581i9586 

0     1     1 

223 

344 

91 

9590  9595 

9600 

9605 

9609 

9614 

9619 

9624 

96289633 

Oil 

223 

344 

92   96389643 

9647 

9652 

9657 

9661 

9666 

9671 

967519680 

0     1 

223 

344 

93   9685  9689 

9694 

9699 

9703 

9708 

9713 

9717 

97229727 

0     1 

223 

344 

94 

97319736 

9741 

9745 

97509754 

9759 

9763 

9768 

9773 

0     1 

223 

344 

95 

97779782 

9786 

9791 

97959800 

9805 

9809 

9814 

9818 

0     1 

223 

344 

96    98239827 

9832 

9836 

9841  9845 

9850 

9854 

9859 

9863 

0     1 

223 

344 

97   98689872 

9877 

9881 

9886,9890 

9894 

9899 

9903 

9908 

0     1 

223 

344 

98  199129917 

9921 

9926 

99309934 

9939 

9943 

9948 

9952 

0     1 

223 

344 

99   9956 

9961 

9965 

9969 

99749978 

9983 

9987 

9991 

9996 

0     1 

223 

334 

373 


374 


APPENDIX 


Atomic  Weights. 
(Revised  to  1914.) 


Aluminium 

Antimony 

Arsenic 

Barium 

Bismuth 

Boron    . 

Bromine 

Cadmium 

Calcium 

Carbon 

Chlorine 

Chromium 

Cobalt    . 

Copper  . 

Fluorine 

Gold      . 

Hydrogen 

Iodine     . 

Iron 

Lead 


! 

3  =  16. 

0  =  16. 

Al 

27-1 

Lithium              .  " 

.     Li 

6-94 

Sb 

120-2 

Magnesium 

•     Mg 

24-32 

As 

74-96 

Manganese         .    ' 

.     Mn 

54-93 

Ba 

137-37 

Mercury             . 

.     Hg 

200-6 

Bi 

208-0 

Molybdenum     . 

.     Mo 

96-0 

B 

11-0 

Nickel   . 

.    Ni 

58-68 

Br 

79-92 

Nitrogen            . 

.     N 

14-01 

Cd 

112-40 

Oxygen 

.    O 

16-00 

Ca 

40-07 

Phosphorus 

.     P 

31-04 

C 

12-00 

Platinum 

.     Pt 

195-2 

Cl 

35-46 

Potassium 

.    K 

39-10 

Cr 

52-0 

Silicon   . 

.     Si 

28-3 

Co 

58-97 

Silver     . 

•     Ag 

107-88 

Cu 

63-57 

Sodium 

.     Na 

23-00 

F 

19-0 

Strontium 

.     Sr 

87-63 

Au 

197-2 

Sulphur 

.     S 

32-07 

H 

1-008 

Tin        .     , 

.     Sn 

119-0 

I 

126-92 

Titanium 

.     Ti 

48-1 

Fe 

55-84 

Uranium 

.     U 

238-5 

Pb 

207-10 

Zinc 

.     Zn 

65-37 

INDEX   OF    SEPARATIONS 


General  methods  of  separation  are  discussed  briefly  in  the 
systematic  section  (p.  165)  under  the  headings  of  the  various 
elements.  The  following  index  gives  references  only  to  the 
separations  which  have  been  more  fully  described. 


Aluminium  from 

calcium  and  magnesium,  228,  250 

iron,  167 

lead,  manganese,  silica,  etc.,  236 

manganese,  166 

silica,  206,  232 


Iron  from 


copper,  239,  243 

lead,  tin  and  zinc,  224 

lead,  manganese,  silica,  etc.,  236 

manganese,  1 66,  186 

silica,  232,  206 


Bismuth  from  lead,  tin  and  cadmium, 
226 


Cac'mium  from 

bismuth,  lead  and  tin,  226 

zinc,  copper,  etc.,  245 
Calcium  from 

iron,  aluminium   and    magnesium, 
228 

iron,    aluminium    and    phosphate, 
250 

silica,  206,  232 
Chromium  from  iron,  83 
Copper  from 

iron,  239,  243 

lead,  cadmium  and  zinc,  245 

lead,  tin,  etc.,  225 

nickel,  153,  221 

silver,  220 


Lead  from 

bismuth,  cadmium  and  tin,  226 
copper,  cadmium  and  zinc,  245 
copper,  tin,  etc.,  225 
other  metals  in  galena,  244 
silica,  etc.,  236 
tin,  223,  226 


Magnesium  from 

calcium,  iron  and  aluminium,  228 

silica,  206,  232 
Manganese  from 

barium  and  iron,  248 

cadmium,  copper,  iron   and    zinc, 

245 

iron,  1 86 
iron  and  aluminium,  166,  236 


Nickel  from  copper,  153,  221 


Iron  from 

aluminium,  167 

calcium  and  magnesium,  228,  250 

chromium,  83 


Potassium  from 

all  other  metals,  200 
calcium,  etc.,  238 
sodium,  204 


875 


376  INDEX  OF  SEPARATIONS 

Potassium   from    silica,    calcium,  etc.,       Tin  from 

234,  238  bismuth,  lead  and  cadmium,  226 

lead,  223 

Silica  from  metallic  oxides,  206,  228,  lead,  copper,  iron  and  zinc,  224 

232,  236 

Silver  from  copper,  220  Zinc  from 

Sodium  from  cadmium,  copper,  iron   and   man- 

calcium,  etc.,  238  ganese,  245 

potassium,  204  lead,  copper,  tin  and  iron,  224 

silica,  calcium,  etc.,  234,  238 


INDEX 


ABSORPTION  apparatus  for  gases,  280 

pipettes,  259 
Acetic   acid,  volumetric  determination 

of,  55 

Acidimetry  and  alkalimetry,  44 
Acidity  of  water,  307,  317 
Albite,  232,  366 
Albumenoid   ammonia  in    water,    293, 

3i6 

Alcohol,  density  of,  370 
Alloys,  analysis  of,  22O 

fusible,  226 

preparation  for  analysis,  17 
Alum,  aluminium  in,  130 
Aluminium,  165 

as  basic  acetate,  165 

as  oxide,  130 

bronze,  224 

Ammonia  (table  of  densities),  370 
Ammonia,  colorimetrically,  159 

direct  determination  of,  59 

indirect  determination  of,  58 

in  water,  293,  316 
Ammonium,  167 

Ammonium  thiocyanate,  standard,  IOI 
Amount  of  substance  for  analysis,  19 
Anorthite,  232 
Antimony  as  sulphide,  167 
Arsenic,  gravimetric  determination  of, 
161 

volumetric  determination  of,  88 
Asbestos  filter,  1 1 3 
Aspirator,  177,  181 
Atmospheric  carbon  dioxide,  282 
Atomic  weights,  table  of,  374 
Azolitmin,  364 

BALANCE,  sensitiveness  of,  7 

use  of,  4,  8 

Barium  as  chromate,  169 
377 


Barium  as  sulphate,  169 

hydroxide,  standard,  61 
Baryta,  standard,  61 
Basic  acetate  method,  165 

slag,  249 
Beakers,  14 
Beckmann  boiling-point  method,  353 

thermometer,  351 
Bismuth,  gravimetric  determination  of, 

170 
Bleaching  powder,  valuation  of,  94,  107 

total  chlorine  in,  105 
Bohemian  glass,  236 
Boiling-point  method,  353,  355,  356 
Bone  dust,  249 

Borax,  volumetric  analysis  of,  55 
Bromide,  gravimetric  determination  of, 

173 

volumetric  determination  of,  99,  103 
Bromine  in  an  organic  substance,  335 
Bronze,  analysis  of,  224 
Brucine  test  for  nitrate,  300 
Bunsen  burner,  use  of,  117 

valve,  26,  75 
Burette,  calibration  of,  37 

clamp,  Ostwald,  63 

cleaning  a,  33 

fitted  for  constant  use,  62 

Schellbach,  37 

use  of,  35 
Burner,  Bunsen,  117 

Meker,  117 

CADMIUM  as  sulphide,  174 

electrolytic  determination  of,  149 
Calcium,  175 

as  oxalate,  143 

carbonate,  properties  of,  143 
purification  of,  234 

chloride  tube,  176 


378 


INDEX 


Calcium  in  water,  306,  314 
hydroxide,  standard,  63 
oxide  in  lime,  57 
oxide,  properties  of,  143 
volumetric  determination  of,  69,  306 
Calibration  of  a  burette,  37 
(use  of  term),  32 
of  weights,  10 
Carbide  method  for  determination  of 

water,  213 

Carbon  dioxide  in  air,  282 
in  gaseous  mixture,  261,  266,  272 
preparation  of  pure,  1 1 6 
Carbon  in  an  organic  substance,  318 
Carbon  monoxide  in  gaseous  mixture, 

263,  266 
Carbonate,  direct  method,  175 

indirect  method,  179 
Carbonate-free  sodium  hydroxide,  54 
Casseroles,  15 
Cassiterite,  212,  367 
c.c.  as  unit  of  volume,  31 
Chlorate,  181 

in  bleaching  powder,  105 
volumetric  determination  of,  92,  104 
Chloride,  181 

as  silver  chloride,  133 
in  water,  298,  316 

volumetric  determination  of,  99,  103 
Chlorine   in    bleaching   powder,   avail- 
able, 94,  107 
total,  105 

in  an  organic  substance,  335 
Chloroplatinic    acid,    see   hydrochloro- 

platinic  acid 
Chromate,    gravimetric    determination 

of,  183 

volumetric  determination  of,  84,  92 
Chromium,  182 

in  chrome  iron  ore,  83 
Clay,  232,  366 

Cleaning  glass  vessels,  33,  48 
Coal  gas,  hydrocyanic  acid  in,  285 
hydrogen  sulphide  in,  285 
sulphur  in,  280 

Coin,  analysis  of  bronze,  89,  224 
analysis  of  nickel,  221 
analysis  of  silver,  220 
Colorimetric  methods,  155 
Combustion  apparatus,  319 
pipette,  267 


Combustion  of  a  liquid,  327 

of  a  solid,  324 
Conductivity  of  water,  291 
Copper,  183 

colorimetric  determination  of,  158 

electrolytic    determination    of,    148, 
ISO 

gravimetric    determination    of,    139, 
141,  183 

volumetric  determination  of,  89 
Copper  as  oxide,  139 

as  sulphide,  141 

as  thiocyanate,  184 

in  water,  313 

pyrites,  analysis  of,  241,  367 
Copper-zinc  couple,  301 
Crucible,  Gooch,  113 

nickel,  114 

platinum,  in 

porcelain,  no 

Rose,  115 

silica,  no 
Crucible  tongs,  112 
Cupferron  method  for  iron,  186 

preparation  of,  363 

Cyanide,  volumetric  determination  of, 
99 

DECANTATION,  washing  by,  25 
Density  of  gases,  371 

tables,  368 

determination  of  vapour,  339,  342 
Desiccator,  15 
Dichromate,  standard,  71 
Dionic  water  tester,  291 
Dolomite,  analysis  of,  228,  366 
Drying  a  precipitate,  Il8 
Dumas'  method  for  nitrogen,  329 

ELECTROLYTIC  methods,  145 
Etching  a  line  on  glass,  34 
Ethylene  in  gaseous  mixture,  261,  266 
Evaporation,  21 

FACTORS,  use  of,  30 

Feather,  trimmed,  25 

Feldspar,  232 

Ferric   salts,  volumetric  determination 

of,  76 

Ferrous  salts,  oxidation  of,  75 
volumetric  determination  of,  76 


INDEX 


379 


Filter  papers,  choice  of,  24 
Filter,  incineration  of,  119 
Filtration,  23 

with  Gooch  crucible,  1 1 2 

with  suction,  26,  112 
Flask,  standard,  30,  32 
Flint  glass,  236,  366 
Flue  gases,  sulphur  dioxide  in,  285 
Freezing-point  method,  345 
Funnels,  15 
Fusible  alloy,  analysis  of,  226 

GALENA,  analysis  of,  244 
Garnet,  232,  367 
Gas  analyst,  253 

burette,  257 

-holder,  320 

pipettes,  259 
Gases,  density  of,  368 
German  silver,  analysis  of,  221 
Glass,  analysis  of,  236,  366 
Gooch  crucible,  use  of,  113 
Gravimetric  analysis,  109 
Grinding  minerals,  17 
Gun  metal,  224 

HARDNESS  of  water,  302,  292,  311,  316 
Hempel  apparatus,  257 
burette,  257 
pipettes,  259 

Hydrochloric    acid,    constant    boiling- 
point,  51 
influence  on  permanganate  titration, 

65 

standard,  47 
(table  of  densities),  368 
Hydrochloroplatinic  acid,  364 
Hydrocyanic  acid  in  coal  gas,  285 
volumetric  determination  of,  100 
Hydrofluoric   acid,    testing   purity   of, 

209 

Hydrogen  in  gaseous  mixture,  266,  272 
in  an  organic  substance,  318 
peroxide,  277 
preparation  of  pure,  115 
sulphide  in  coal  gas,  285 

volumetric  determination  of,  91 
Hypochlorite,  volumetric  determination 
of,  94,  107 

IGNITION  of  precipitates,  116 
Incineration  of  filter,  119 


Indicators,  30,  44,  364 

Iodide,  volumetric  determination  of,  103 

Iodine  in  an  organic  substance,  335 

purification  of,  86  {footnote) 

standard,  85,  88 
Iron,  185 

colorimetric  determination  of,  156 

gravimetric    determination    of,    127, 
185 

volumetric  determination  of,  74,  76, 

81,  83 
Iron  as  basic  acetate,  165 

as  oxide,  127 
Iron  in  black  ink,  83 

in  chrome  iron  ore,  83 

in  ferric  salts,  76 

in  iron  alum,  125,  127 

in  iron  wire,  74 

in  a  mineral,  80,  82 

in  water,  313 
Iron  pyrites,  analysis  of,  239,  366 

KAOLINITE,  232 

Kjeldahl's  method  for  nitrogen,  334 

LANDSBERGER    boiling-point    method, 

356 

Lawrence  Smith  method,  234 
Lead,  187 

action  of  water  on,  312 

colorimetric  determination  of,  161 

electrolytic  determination  of,  153 

gravimetric  determination  of,  187 
Lead  in  water,  311,  317 
Lead  peroxide,  valuation  of,  92 
Lime,  determination  of  solubility  of,  56 

volumetric  analysis  of,  57 

water,  standard,  63 
Limestone,  analysis  of,  228 
Litmus  solution,  preparation  of,  364 

use  of,  45 

Lubricants  for  glass  taps,  35  {footnote) 
Lumsden's  vapour  density  method,  342 
Lunge  nitrometer,  275 

MAGNESIA  mixture,  363 
Magnesium,  188 

as  pyrophosphate,  135 

in  water,  306,  314 
Manganese,  189 

colorimetric  determination  of,  162 

gravimetric  determination  of,  1 89 


380 


INDEX 


Manganese  dioxide,  volumetric   deter- 
mination of,  67,  92 
analysis  of  crude,  248,  367 
Manganese  in  steel,  163 
Manganite,  analysis  of,  248,  367 
Meker  burner,  use  of,  117 
Mercury,  192 

gravimetric  determination  of,  192 
volumetric  determination  of,  56,  104 
Metals,  preparation  for  analysis,  17 
Methane  in  gaseous  mixture,  266,  272 
Methyl  orange  solution,  preparation  of, 

364 

use  of,  45 
Methyl  red  solution,  preparation  of,  364 

use  of,  46 
Mica,  232 

Minerals,  preparation  for  analysis,  17 
Molecular    weights,    determination    of, 

338 

Mortar  for  hard  minerals,  17 
Muscovite,  232 

NESSLER  solution,  159 

tubes,  156 

Newton's  alloy,  analysis  of,  226 
Nickel,  195 

electrolytic  determination  of,  153 

gravimetric  determination  of,  195 
Nickel  coin,  analysis  of,  153,  221 

copper  in,  89 
Nitrate,  60,  70,  275 

in  water,  300,  317 

volumetric  determination  of,  60,  70 
Nitric  acid  (table  of  densities),  369 
Nitrite,  68,  275 

in  water,  299,  316 

volumetric  determination  of,  68 
Nitrogen  in  gaseous  mixture,  266 

in  nitrate  or  nitrite,  275 

in  an  organic  substance,  329,  334 
Nitrometer,  Lunge,  275 

Schiff's,  330 
Normal  solution,  definition  of,  29 

ORGANIC  analysis,  318 

Orsat  apparatus,  269 

Orthoclase,  232,  366 

Ostwald  burette  clamp,  63 

Oxalate,  volumetric  determination  of,  67 

Oxidation  and  reduction  processes,  29 


Oxidation  of  ferrous  salts,  75 

Oxygen  in  gaseous  mixture,  262,  263, 

266 
preparation  of  pure,  267  {footnote) 

PAPER  mats,  use  of,  16 
Parallax,  error  due  to,  36,  37 
Penfield's  method  for  water  in  minerals, 

215 
Perchloric  acid,  363 

(table  of  densities),  369 
Percussion  mortar,  17 
Permanent  hardness  of  water,  303,  304 
Permanganate,  standard,  64 

titrations  in  presence  of  hydrochloric 

acid,  65 

Peroxides,  determination  of,  67,  92,  277 
Persulphate,   volumetric  determination 

of,  6 1 

Phenolphthalein   solution,   preparation 
of,  364 

use  of,  46 

Phosphate,   gravimetric    determination 
of,  196 

in  water,  302 
Pipe-clay  triangle,  112 
Pipette,  31 

standardisation  of,  39 

use  of,  39 

Platinum  crucibles,  use  of,  ill 
Potassium,  199 

in  glass,  238 

in  insoluble  silicate,  234,  238 

pyrosulphate,  fusion  with,  8 1 

separation  from  sodium,  204 
Potassium    cyanide,   volumetric   deter- 
mination of,  99 

Potassium  dichr ornate,  standard,  71 
Potassium  hydroxide  solutions  (table  of 
densities),  370 

vapour  pressure  of,  371 
Potassium  permanganate,  standard,  64 
Potassium  thiocyanate,  standard,  101 
Precipitates,  drying  of,  1 1 8 

ignition  of,  116 

washing  of,  25,  128 

Precipitation,   general   instructions   re- 
garding, 22 

Pressure  regulator  for  filter-pump,  114 
Purification  of  salts,  1 6 
Pyrites,  analysis  of,  239,  241,  366,  367 


INDEX 


381 


Pyrolusite,  analysis  of,  248,  367 

valuation  of,  67,  92 
Pyrosulphate,  fusion  with,  8 1 

REAGENTS,  list  of  common,  361 

used  in  gas  pipettes,  261 
Recrystallisation  of  salts,  16 
Red  lead,  valuation  of,  92 
Reduction  of  ferric  salts,  76 
Rocks,  preparation  for  analysis,  17 
Rose  crucible,  use  of,  115 
Rose's  alloy,  analysis  of,  226 
Rotating  electrode,  150 
Rubber,  permeability  to  gases,  256 

SAMPLING  a  gas,  254,  258,  271 
Schellbach  burette,  37 
Scoop  for  weighing,  18 
Silica,  206 

in  insoluble  silicate,  206,  232 
in  water-glass,  210 
Silica  crucible,  HO 
Silica  plate  for  excluding  flame  gases 

from  crucible,  112 

Silicate,  analysis  of  insoluble,  232,  366 
Silver,  gravimetric  determination  of,  2 1 1 

volumetric  determination  of,  104 
Silver  coin,  analysis  of,  220 
nitrate,  standard,  98,  IO2 
Soda-lime  tube,  177 
Sodium  199 
in  glass,  238 

in  insoluble  silicate,  234,  238 
separation  from  potassium,  204 
Sodium  arsenite,  standard,  88,  107 
Sodium  hydroxide,  carbonate- free,  54 
(table  of  densities),  370 
preparation  of  pure,  140 
standard,  52 

Sodium  sulphide,  standard,  108 
Sodium  thiosulphate,  standard,  85,  90 
Soft  water,  286,  302,  311,  316 
Solder,  analysis  of,  223 
Standard  flask,  JO,  32 
pipette,  39 

solutions  (general  notes),  29  41 
solutions  of  desired  concentration,  43 
Standardisation  (use  of  term),  32 
of  a  flask,  32 
of  a  pipette,  39 


Starch  solution,  86 
Steam- bath,  22 
Steel,  manganese  in,  163 
Stirring-rod,  15,  25 

Sulphate,  gravimetric  determination  of, 
131,  212 

in  copper  sulphate,  58 
Sulphide,  91,  212 

standard,  108 
Sulphite,  volumetric  determination  of, 

91 
Sulphur  in  coal  gas,  280 

in  an  organic  substance,  336 

in  minerals,  240,  242 
Sulphur  dioxide  in  flue  gases,  28$ 
Sulphuric  acid,  standard,  52 

(table  of  densities),  368 
Sulphurous  acid,  volumetric  determina- 
tion of,  91 
Superphosphate,  analysis  of,  249 

TALC,  366 

Tare  of  a  crucible,  119 
Temporary  hardness  of  water,  303 
Thiocyanate,  standard,  IOI 
Thiosulphate,  standard,  85,  90 
Tin,  212 

volumetric  determination  of,  9$,  96 

UNIT  of  volume,  31 

VALVE  for  wash-bottle,  26 

Vapour  density,  determination  of,  339, 

342 

Vapour  pressure  of  potassium  hydrox- 
ide solutions,  371 
of  water,  371 

Victor  Meyer's  vapour  density  method, 

339 

Vinegar,  acetic  acid  in,  55 
Volume  of  I  gram  of  water,  32 
Volumetric  analysis,  28 

WALKER-LUMSDE.N  boiling-point 

method,  356 
Wash-bottle,  14 

Washing  of  precipitates,  25,  128 
Water,  acidity  or  alkalinity  of,  307,  317 

action  on  lead,  312,  317 

ammonia-free,  160 


382 


INDEX 


Water,  ammonia  in,  293,  316 
analysis,  286 
calcium  in,  306,  314 
chloride  in,  298,  316 
colour  of,  291 
conductivity  of,  291 
copper  in,  313 
gravimetric    determination    of,    123, 

124,  213 

hardness  of,  302,  292,  316 
iron  in,  313 
lead  in,  311,  317 
magnesium  in,  306,  314 
nitrate  in,  300,  317 
nitrite  in,  299,  316 
odour  of,  290 

organic  matter  in,  297,  294,  316 
phosphate  in,  302 
salts  in,  313 
solids  in,  292,  316 
softness  of,  286,  302,  311,  316 
taste  of,  291 


Water,  total  solids  in,  292,  316 

turbidity  of,  290 

vapour  pressure  of,  371 

zinc  in,  313 

Water-glass,  silica  in,  210 
Weighing,  method  of,  5 
Weighing  "by  difference,"  18 
Weighing-bottle,  18 

-scoop,  1 8 
Weight  of  I  c.c.  of  water,  32 

of  substance  for  analysis,  19 
Weights,  calibration  of,  lo 
Wood's  alloy,  analysis  of,  226 

ZINC,  216 

as  oxide,  137 

gravimetric   determination    of,    137, 
216 

volumetric  determination  of,  108 
Zinc  blende,  analysis  of,  245,  367 
Zinc  dust,  valuation  of,  278 
Zinc  in  water,  313 


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